An old farmer said when troubleshooting electrical issues the 3 most important things are to:
Check your grounds
Check your grounds
Check your grounds
I think he sold real estate on the side. 😉
So, check all connections, meaning take them apart and clean them. If all good there, next step is to check the battery.
Cheap and easy way is to remove the solar charger and check voltage, it better be above 12, closer to 13 is better.
Now with the voltmeter still connected, hit the starter and watch the voltage. If it drops below 10 you have a battery problem. Try jumping with another vehicle.
If battery voltage is good, check again with the voltmeter connected at the starter. If OK, time to remove the starter and have a look inside. If voltage drops off at starter but is OK at the battery, you have a cable/connection problem.
Let us know what you find.