18w per panel seems still way too low.
To work out if the problem is on the panel side or the battery side, you really have to put one panel on the cables that go out to the array, take the leads out of the MPPT, and short them out. This is the only real test that you don’t have a bad connector somewhere.
Were your MC4 connectors crimped with the correct tool?
See this post about badly crimped connectors; This is why you use the correct tools
Once you have a Voc test (open circuit voltage) and an Isc (short circuit current) test done, you can move on to the battery and settings side