So this is what I did for a long time at my folks place out in the boonies.

1. Get yourself another line with unlimited data.
2. Buy yourself one of these: GL.iNet GL-MT3000 or GL.iNet GL-AX1800
3. Connect the phone to the USB slot.
4. Turn on the phone’s USB tethering option.
5. Go into the router’s admin page and tell it to use USB tethering as the WAN option.