It sounds like things aren’t connected to the Mayfly board properly. The 3.7v Lipo battery gets connected to only one of the JST jacks labeled LIPO BATT on the Mayfly board, then a 6v solar panel gets connected to only one of the jacks labeled either SOLAR1 or SOLAR2, and not both. Are you using one of the old style glossy 3.5w panels, or one of the newer 5W ETFE panels with the textured matte finish? They are in stock: https://voltaicsystems.com/5-watt-panel-etfe/
What happens if you unplug the solar panel from the Mayfly board, what kind of reading to you get? And have you tried swapping the battery pack for another one?