I'm interested in this as well. My Comcast cable goes down fairly frequently late at night for "scheduled maintenance" that they never notify about, despite my requests over many years. I'm tired of calling them to request bill credits each time. And that doesn't solve the problem of interrupted streaming. I need a cheap backup ISP for about 4-10 hours a month worth of Comcast downtime.
I have been testing Verizon 5G Home Internet since Saturday, because they upgraded their towers in my area, and offer the service at my address. The other carriers (AT&T, T-Mobile) will not sell it to me, for good reason - weak signal. I setup the Verizon ARC-XCI55AX gateway in IP passthrough mode. It is a perfectly adequate backup. The basic service, which I'm testing, costs $50/month, however. $600/year is a bit much to pay to cover the 4-10 hours worth of Comcast downtime that they shouldn't have in the first place.
It just so happens that my cellular provider, US Mobile, is an MVNO for both T-Mobile or Verizon. They offer a shared data plan, which would be perfect for our 2 smartphones, and a 3rd line for the home ISP backup. However, I need an unlocked 5G modem that will accept either an eSIM or a physical SIM. And it needs to preferably connect to pfSense via Ethernet. So far, I have not been able to locate such a device. Is there one ? One device was mentioned upthread, but it is only 4G LTE, not 5G, and the manufacturer does not have a 5G version.
The other possibility would be to use a smartphone for the backup WAN. I have not tried to enable USB tethering on my phone and connecting it to pfSense. Right now, the phone is still on the T-Mobile network, which provides speeds of 0.07 Mbps up and 0.04 Mbps down. Definitely not suitable as a backup WAN.
If USB tethering doesn't work, the other option would be to use the phone as Wifi hotspot, and add a Wifi NIC to pfSense for the WAN connection. It seems that this should work per https://docs.netgate.com/pfsense/en/latest/wireless/configuration-wan.html, although it does not mention how to input the Wifi password. Presumably this is part of the encryption settings.
I would much prefer to use a 5G modem with an Ethernet connection if one exists, than use a smartphone, though. Phones are subject to getting updates and rebooting themselves at the most inopportune times, possibly requiring user interaction to restart, which is not always possible if I'm traveling and need to access the home VPN. Phones also depend on a battery with limited lifespan.