I am currently living with my parents and we have just started an Internet contract with a 5G wireless company.
The issue is the MFND settings are behind a password and likely not allowed access by the ISP. Even if they weren't doing port forwarding on 5G likely isn't possible because of CGNAT. I think I can use cloudflare tunnels or tailscale to get around this, and not many things need to be directly accessible from the Internet.
The more annoying thing is that setting DHCP reservations likely isn't possible without getting access to the settings. It's going to make setting up static IPs difficult too.
Before anyone asks fixed line Internet almost certainly isn't practical in this area. Getting our own modem while possible is more expensive and potentially difficult, and would mean cancelling the contract.
Is there a reasonable way to work around these issues?
Do you even need reservations? You can also just set a static IP on the computer and it should be fine. Most DHCP servers test the IP before handing it out just in case.
Most, but not all, do. So it might be as simple as setting a static address, or it may overlap in the future.
You could ask from ISP (or try it out yourself) if you can use some addresses outside of DHCP pool, my ISP router had /24 subnet with .0.1 as gateway but DHCP pool started from .0.101 so there would've been plenty of addresses to use. Mine had a 'end user' account too from wehere I could've changed LAN IP's, SSID and other basic stuff, but I replaced the whole thing with my own.