Well, for basic WiFi to dramatically extend coverage, you would need to take a page from the WISP/mobile carrier playbook and set it up with semi directional panels, in a configuration that covers 365 degrees. Placing the units high up on a tower, angled down slightly, with their power levels as high as they can go.

Primarily using 2.4Ghz would provide better penetration of signals in the area, but there would also be more conflicts.

Still, as I've said, even in this configuration, you may get a few blocks of coverage at best... At least to a regular device with an Omni antenna, like a phone or laptop. The only way to extend any further would be to use directional antennas on both sides, which is what WISP companies do. In that case, you can generally transmit to anywhere in line of sight of the base antenna system. Being very high up vertically becomes a more significant factor since there's plenty of buildings, trees, and other obstacles that are going to block the communication. As I mentioned, at extreme range, you're going to contend with the curvature of the earth itself, so that's something to bear in mind when going very very far with it.

Regulations allow for the public to use 2.4/5 GHz as well as 900mhz and, I believe, 430? MHz? There's higher frequencies like 60Ghz, but at that wavelength, you start to see scattering and attenuation from atmospheric events like rain and fog. Those things still affect 2.4 & 5Ghz, but usually not significantly enough to cause a failure in the link. The ISM 5.8ghz band may be a good place to go for high speed data, but for simple (slow) data, 2.4 GHz will be more reliable.

The regulatory bodies that govern frequency use are fairly strict when it comes to what can be done without a license. The only way to guarantee that you can do pretty much whatever you want would be to request a frequency for your own uses from your local regulatory authority (in the US, it's the FCC, in Canada it's IC or industry Canada, other countries will have similar government organizations to manage the EM/RF airspace). With a license you may have power restrictions based on the area you intend to serve (they don't want you blasting the signal so strongly that you encroach on other licensed people legally using their system in their area), but you should be free to use any mode (analog, digital, data, etc), with any level of encryption you want, and grant permission to anyone else to use the frequency in your designated area. This is how mobile providers operate, but usually they're buying large bands of frequency ranges for use across the entirely of a state/province/country, so their pricing is well into six figures. It's not cheap to get such licensing, likely well into the thousands per year, even for a modest area with a small frequency range. Additionally, your name goes on the license, and that's usually made available to the public, though few ever actually look at that data, it is available nonetheless. You can otherwise anonymize the transmissions and encrypt the messages to your heart's content. The challenge there is that everything that can transmit on those licensed bands is extremely expensive; since usually only businesses with direct need for such systems ever apply for a license like that. The only places I've known to use a licensed RF system have been physically large places, like airports (security, airlines etc.), large campus style institutions like universities and colleges, and parks services (even as small as water parks) many smaller businesses are illegally using FRS and GMRS for in-store communication; places like your local hardware store may do this. Larger franchise places are a mix of purpose built communication systems, like with the headsets commonly used by fast food restaurants, which are very short range and that is counterbalanced by the illegal use of residential systems like FRS for the same purpose.

Unlicensed residential communications like FRS are usually very short range and very power restricted, and the legislation (at least here) usually stipulates that they are only to be used for personal communication, not professional/business communication; so the use of such services in a commercial environment is illegal. However, enforcement is usually not very strict, and businesses do it anyways, and likely don't get punished for such abuse of those systems because nobody is enforcing the rules against it. Businesses know that enforcement is poor and often disregard the potential risks of using those services illegally.

I know ham operators are generally very adamant about people following the rules and I've heard many stories of individuals thinking they can just use ham bands without proper identification or qualifications without consequence, much the same way as businesses do with FRS, but licensed operators have and do, track down such use and report it, and that's about the only time I've heard that people get punished for their illegal use of frequencies and bands... when it is reported directly to the FCC/IC/whomever is in charge of the allocations. The only instances of reports that I've heard are from ham operators and city services like fire departments when you illegally broadcast on their channels.

Most of the unlicensed frequencies are generally the wild west of radio, and all manner of strange and unusual happenings occur on those bands. It's usually few and far between that you'll experience it, but it happens and often, nobody has the skill or care, to bother reporting it. Nonetheless it is illegal to broadcast over the frequency power limits and the penalties for being caught are steep, up to and including jail time; so it's not something I would ever endorse. Regardless, you have been warned. Check your local laws from your regulatory authority and ensure you are complying with laws before turning on any transmitter (listening to any/every frequency is generally not restricted, though, understanding the transmissions is usually an issue, especially with digital/encrypted communications on licensed frequencies). YMMV on this.

Regardless of all of that, if you wish to pursue an emergency or wide spread communications system for data, including encrypted data, I wish you the best of luck. I'm happy to lend a hand and experiment along with you if you desire it. I've always felt that exclusively relying on primary communication systems, like mobile phone systems and public switched telephone networks (things that otherwise rely on infrastructure) is an inherent weakness. I'm an IT person by day, with specialty in wireless and networking technologies, so you've struck a chord here. I'm very interested in any data communication that can be used during a primary infrastructure failure. Personally I experienced the full force of such a failure a few years back when one of the major telecommunications providers in my country had a significant/system wide communications break down for more than 24 hours. I was a mobile subscriber to their service, and for that day, I was unable to use any data services on my cellphone, nor make our receive any calls. It was a wake up call that if there's a large scale failure of the system I relied on, then I would be dead in the water in terms of being able to get help in an emergency. At the time I didn't have my ham license, and I was unable to even check and make sure my loved ones were okay while I was away from a working WiFi connection. At the time I was traveling to work, and while on the road I was entirely communications blind. Something I haven't experienced for nearly 20 years. Unfortunately, I'm the only person in my family to have achieved my amateur qualification, but I can at least request that someone with a working connection call and talk to someone (over the radio), if such an event reoccurs. Having a method to communicate with them in the event of another such failure without requiring them to go through the amateur qualification would be something very desirable to me.