As long as you're certain your wireguard server works from other networks.. You could use nginx as a proxy to have your server accept incoming traffic from UDP port 53 (DNS)... But, be careful!
I would strongly suggest you whitelist the retrictive network for use on UDP port 53. Is the WAN IP static on the restrictive network? Do you know the size of the WAN subnet? I'd try a quick test over UDP port 53 using nginx, then close it up. If it works, create your firewall rule to whitelist only the restrictive network and Bob's your aunt.
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