Hello,
From your information you need router with WAN failover function. Main WAN would be WiFi and backup WAN would be mobile yes? If so then RUT240, RUT950 and RUT955 can be used, they are LTE routers, they have from 2 to 4 Ethernet ports (RUT240 x2 Ethernet ports RUT9xx x4 Ethernet ports). To try and achieve what you have described simple open RUT WebUI and navigate to Network -> WAN, select wireless WAN (WiFi) as main and mobile WAN as backup. After that simple scan wireless network find your harbor WiFi and connect router to it, router will have internet access (and will provide internet access) via harbor WiFi, and now if you would to sail to the sea router would lose wireless connection to the harbor WiFi and will automatically switch to backup WAN (mobile) and also vice versa when sailing back to harbor when wireless network would be in the range of RUTxxx wireless network router should connect to it (switch back from mobile WAN to wireless WAN).