It's a good idea to set the timezone before installing the license key so the messages in the log have the correct timestamp in case you run into a problem.
Also in your case with 2x C5210 in a pair it's mandatory to set an extrenal NTP server so the controller pair is in sync.
V9 of the controller software introduced the license pool feature = the controller availabilty pair share the licenses into one pool.
So in your case it doesn't matter on which controller you activate the AP licenses, they will share a pool together as soon as availabilty is activated.
You'd find the controller firmware on the Exrtranet....
https://extranet.extremenetworks.com/downloads/Pages/WirelessControllers.aspx
I don't run into any major issue with the latest code V9.21.01.0179.
If you don't have an account please contact the partner who sold you the hardware.