First of all, USB modem support by BSD is not the best, maybe you should search for specific device to see if it will work under BSD.Then I guess you will do good not to block private networks, as cellular mostly is CGNAT.I never used USB LTE modems, but i guess you need to set up PtP connection in your sense (providing APN, etc.). Have you done this and missed to provide screenshots?
ue0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500 description: OPT2 (opt2) options=80000<LINKSTATE> ether 34:4b:50:00:00:00 media: Ethernet autoselect status: active nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>