I suspect that as long as the system "thinks" the maintenance has expired, then it is frozen.
You should look at the activation page to get the system's understanding on your status. If you are certain that the maintenance is still valid, then you can try and reactivate the license which should then validate with 3CX and reset to the valid status and show the new expiration date.
If the condition remains expired, then you should contact the reseller who renewed or 3CX if you renewed directly with them.
3CX has elected to provide a free upgrade for V12 users to get to V12.5 for those whose maintenance has expired. This will get you around the certificate issue, but once done you will still be frozen at this level. You will need to do a back-up of your current system, uninstall the V12, download and install V12.5 and then restore. I think this month is the final month of the offer. I just upgraded a "new inherited" client last week who was in the same position as you.
