It's not pretty but last year we used a
3G USB router connected to a little Ethernet switch to serve wired Internet to all of our scouts who didn't have a 3G iPad. We plugged a Verizon 3G USB dongle into the router, turned off the router's WiFi, and it all worked like a champ. We ran the whole thing off an Inverter powered by a standard robot battery for those cases where we couldn't plug it in.
Verizon and AT&T will let you register a 3G dongle month to month as long you own it. This year we are using
this one because we just need a single laptop with everyone else using 3G iPads. I don't know if that dongle will work on the Zoom modem.