What cell phone companies cover your location?
Is there more than one cell phone company covering the area?
Would 3g (400 kbps) be fast enough?
Is 3g still available on the local tower?
There are routers which will combine more than one source of internet boosting the speed. Cradlepoint makes one (or more). There are some folks offering 3g devices with unlimited data at $15 per month.
There should be no need to use the internet bandwidth for the phone. Use a cell phone for voice only.
T-Mobile does offer unlimited data with 4G LTE in Palmer. Does it cover the campground?