the top ribbon cable is for the SR and SL buttons and the Sync status. When you disconnect the joycon the green LED’s will flash and then go solid when connected to the switch.
The bottom ribbon cable is for power and handshakes along with button presses.
The joycon rails and the console rails could be dirty/corroded. You are getting the charge symbol when plugged in so that means at least some of the pins are connected.
If the ribbon cable for power/handshakes is damaged you will sometimes get this fault.
the way it works for reference is the joycon advertises it is available via the pins when connected and the switch plays the sound and animation however for the first time connecting it will show as grey instead of the actual colour of the joycon. After a second the console will show the joycon connected and its colour as the eprom the the joycon shares that information along with serial number/mac
if it was working before you swapped shells then chances are its a bad connection on the bottom ribbon cable or the ribbon cable is damaged. If these are good then check the pins on the console and the joycon for dirty/corrosion. An easy test is to use a different joycon on the console to see if it is seen.