A solar panel
is a fairly simple device really.
They either work or they do not.
Individual cells can weaken and load down the collective output but you are describing the voltage as where it should be.
You can connect a meter directly to the panel indoors and then go cell by cell with a flashlight and determine roughly which cells work and which ones may be weak.
I think though that if you are getting 18v out of the panel it sounds OK.
You didn't mention how big a panel it is and what exact control you have?
Some controls will shunt the panel output to avoid overcharging and some are very inexpensive and would be easy to replace.
If you connected the panel to the control but then no battery
to the control it may not output voltage either without an output load on it.
It sounds to me that you may have found the problem though already?