Since this is a unit you purchased online I would recommend you do the following:
1 - Reset it back to factory default (https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-factory-reset-WiNG-AP650-or-AP6532)
2 - Check firmware version on AP and make sure it's not too far behind (Please provide version)
3 - Check firmware on controller to make sure it supports the AP (Please provide version)
3 - Make sure you have enough licenses on the controller (#show lic)
4 - If using layer 2 adoption on the controller, verify that Mint MLCP VLAN is not disabled (on both #self >> #show mint mlcp)
5 - Run command '#show mint neighbors' on both, what do you get?
What method do you use to adopt AP:
DHCP option 191, Controller host, MLCP VLAN, etc.?
Thank you,
Chris
Christoph S.