We have seen low power issues cause multiple problems and I would not completely rule it out. Try direct connecting one of the units to your boat’s cranking battery and see if the GPS will work than.
The Ethernet connection should not make a difference.
If you know of someone else with a Humminbird unit that has an external GPS Receiver, you could try testing one of your units (or both) on their boat to see if the GPS will work there or not.