What kind of internet do you have? Fiber, DSL, or cable? I'm also wondering how your TV is set up as well. Is it hooked through a coaxial cable right into the modem? A picture of the modem might help.
Cable tv shouldn't require a modem under most circumstances. Unless you have fiber optic TV, which then converts digital signal of fiber back to analog signal through coaxial cable, which then hooks into your TV?
That is normal behavior when you hook one router into the ISP router without disabling routing capabilities with the ISP router first. I've experienced this very same thing.
Sounds like you have a modem/router from the ISP. You have to disable routing capabilities on the ISP device (Basically bridge mode). Then all routing can be left up to one of the Asus routers. I would put one of the 2 Asus routers into AP or Bridge mode as well. I don't even know if you can set redundant routing or fail over with those anyways. Asus routers might be able to mesh together to keep one SSID across both devices.
Before going too far with this, I'd make sure your ISP (internet service provider) is moca certified. Mine no longer is, so moca was not an option for me.
From the sounds of it, you are using an AIO networking device, which combines routing, switching, and modem functions all in one. So yes, you are correct. Internet speeds AND local speeds between devices would be throttled to 1G.
If you want to make the switch to a full 10G network, which is really nice for up to 10Gbps internet and large local transfers, it'd be best to build a custom pfsense or opnsense router with 10G SFP+ NICs. Also, to "send out that speed" to all other devices locally, it'd be best to get a 10G SFP+ switch so that the router doesn't do all the work. Personally, I have an 8 port Mikrotik SFP+ switch which works great for me.
Main point is, with a 10G network setup, you can get SSD speeds over the network for large file transfers like 4k movies if you want to upload to a media server. 10Gb/s (gigabits per second), or 1.25GB/s (gigabytes per second) is more than double 2x what most SATA SSDs get around 500MB/s. So you can even get PCIE 3.0 NVME speeds over the network, which is pretty cool.
Also, here's some copy paste from forums I found on how to out your isp router into bridge mode, in case you truly need it for TV:
Select ADVANCED SETUP Select PROCEED at advanced user challenge screen Select WAN IP ADDRESSING under the IP ADDRESS section on the left-hand menu. In SECTION 2, Select RFC 1483 Transparent Bridging Select Apply to save any changes at bottom of screen. The "Internet" light on the front of the modem should go solid RED. This indicates the modem is in Bridge mode.