Sorry Francisco but that sounds like rubbish to me.  I have not worked with 5508 myself but I have never heard of a WLC license being linked to AP serial numbers (anybody else know otherwise?).  The license determines the number of APs which can be j...

Also check that the AP isn't getting WLC details from DHCP option 43 or DNS. And if all else fails completely re-flash the AP using the process and u-boot loader at https://www.cisco.com/c/en/us/support/docs/wireless/catalyst-9120axi-access-point/217...

It would require a license RMA but because of end of support it's expected that they will refuse to help. Probably your only option in that case would be to find a 2nd hand controller with enough licenses to support your APs.