We had the same situation as you, great
service up on deck but not down in the
cabin, and we tried several approaches. In the end, we settled on a
cheap wifi range extender from Best Buy for
service in the marina and use a cellphone hotspot for service when underway or at
anchor. Works great. The range extender is plugged in up on deck at the flybridge. It relays a signal down to the
cabin and is five bars all the time. At
anchor, our cellphone hotspot even allows streaming NetFlix and other services to our TV. Working well for us but your mileage may vary.