FYI, for those who want to follow in my footsteps and try to use a US Mobile Warp 5G SIM (Verizon network) and smartphone for backup WAN, I was not successful at doing that. When at home, my unlocked Samsung Galaxy Ultra 5G, originally for T-mobile, gets 4 bars of GSM (voice/SMS) signal in my home office, but no data whatsoever, whether 2G, 3G, 4G or 5G. Meanwhile, the Verizon 5G Home Internet gateway, model ARC-XCI55AX, gets a perfectly good 5G signal, and is perfectly usable over gigabit Ethernet with pfSense, getting peak speeds of about 105 Mbps down / 12 Mbps up.
The only explanation I have for the gateway having a good data signal, but not the phone, is that they must be operating on different bands. The 5G gateway admin page unfortunately does not list the band, just signal strength.
It's unclear if the issue with the phone is the phone not supporting all the same 5G bands that the gateway does, or if the MVNO is deprioritized and not allowed on those bands. But the result is the same - I cannot use the phone for WAN with pfSense, at least not if it's located in the same room. The phone does get a weak 5G data signal in other rooms, and could potentially be used over Wifi, if I added a Wifi NIC to my pfSense box. But the 5G speeds I achieved on the phone of 1-3 Mbps are so low that it's not really a viable backup ISP. No streaming possible for sure with that setup.
I did get a much better 5G signal with the phone in other cities earlier today - achieved 100 Mbps down / 10 Mbps up. Way better than 0 / 0 at home.