The idea is to combine two routers: The Wifi router only works with the Wifi of marinas or cafés. It has an own Web interface and an ethernet port. The LTE router can switch between Internet over cellular GSM and Internet over LAN cable.
Public Wifi
⬇ long distance Wifi
Boat Wifi antenna
⬇ USB
Boat Wifi router
⬇ Ethernet
LTE router (switchable to Internet over LAN)
⬇ Wifi
All devices in the local network
This setup is described in Boote Blog by Julian for their own setup using a TP-Link MR6400 LTE router and an Ubiquiti Bullet M2 Wifi router.
Someone called Cornelia is using the exact same setup using an AVM Fritzbox instead of the TP-Link router.