Perhaps a bit off the wall, but if the objective is an email out, are GSM signals strong enough even at another high point which does afford radio line of sight to the objectives?
Used this many years ago for remote monitoring, in one case the GSM atop a 5m pole with a wind generator attached topping up a pair of batteries, so completely self sufficient. Links to the metering points were low power RF, the landowner had no objections.
Only problems encountered were vandals climbing the pole, solved by a good coat of Copaslip , and a Stag which regularly danced on a GRP cover to a flow meter chamber, to the great amusement of the gamekeeper
