Not a complete answer, but SOME cellular carriers block any incoming traffic, so DynDNS (etc) won't work.
Even if you pay them for a fixed public routable IP, some cellular carriers block certain traffic, like no incoming/mobile-terminated HTTP/port 80 traffic - you'll need to move the port from 80 to something like 8080.
Check what is the IP being assigned, if 10.x.x.x or 172.x.x.x, then you don't have a routable IP. My own experience, if your cell network uses DHCP, you will NOT have a routable IP ever, you will never be able to ping or connect to the WAN 3G from outside.
Over cellular, stock Digi Realport drivers probably won't work, because the timeouts will be too low. You will need to dig down into the Realport settings and add larger (10+ second) timeouts to survive cellular lags.