We use a CradlePoint CTR35 (I think) router (CradlePoint CTR35). It's very compact, AC powered, accepts our USB 3G device, and broadcasts WIFI. We keep the router in our camper's bedroom and it broadcasts around our whole site.
When I set it up, I configured the router to request a password from all devices connecting to it. After we've connected the device, it functions just like our home router.
I have the same Cradlepoint router. Works very well with our air card. Last fall we got a smartphone that supports hotspot. This has been a better and cheaper setup than our air card. Still use the Cradlepoint to retransmit the signal at times. You may also want to look at a Verizon MIFI. You can have up to 5 connections on it.
We use a similar setup as "MegaJohn", but a different brand. We use a pay as you go USB Stick from Verizon that plugs right into the router, and we have a external extended range Wi-Fi antenna with built in Wi-Fi adapter that connects to the router via USB. I mount the Antenna on a short stick of 3/4"pvc and place that into a 1" pvc that is clamped to the ladder, and run the cable in via the slide. I get a boost to 100% on 75% of the signals I see. I can reach 3 to 5 miles with that antenna. For security, I use Avast Security Suite that comes with it's own secure browser when needed for banking and the such. Out of 12 months, I've used free wifi 10 months. Hope this helps, good luck.
2002 Ford F-450 Crew Cab
2013 Heartland Landmark San Antonio
You can purchase a hotspot instead of the USB stick- same function, but the hotspot allows you to connect up to five devices by wifi to the Hotspot. Most providers that sell the usb stick also sell the hotspot box. There are also some Android phones and Iphones that are enabled to do the same thing using the phones data service.