The idea is very simple: A high gain wifi antenna and router like Ubiquiti Bullet M2 is mounted on the boat or camper. Its web interface is used for connecting to distant wifis.
A smartphone receives the GSM cellular network and works as a wifi hotspot in tethering mode. When this network is needed, the only thing to do is calling the Bullet’s web interface and switching to the phone’s network.
Cheap phone in tethering mode
Boat Wifi router (switchable between phone or other wifis)
All devices in the local network