@cmeu What kind of custom firmware do you use? Is that #openwrt - if not consider it.

Check with your FTTN provider - Are they using CGNAT or any other IP4 sharing tech - if so you will need to see if they will do you a static IP or otherwise except you from IP sharing.

At least outside the US most ISPs will let you use your own router with their Fibre modem but if not you need to know how to get their device into bridge mode or at least forward some ports to your own devices.