Cisco WLC 9800 officially support 2700 series but sometimes need to additional configuration.
when an access point tries to connect to WLC it checks the MIC license on WLC and if it expired or mismatch, the DTLS tunnel cannot establish.
the solution is bypassing this step in ap join process.
now you need to do only these 4 steps:
1- connect to WLC 9800 via SSH
2- dive into config mode by Config Terminal command.
3- ap cert-expiry-ignore mic enable config
4- ap cert-expiry-ignore ssc enable save config
Good luck…