Hi,
As you are probably aware, Initially both AP’s to be used in mesh link have to be wired and in the same subnet. Ensure the basic configuration has been synchronised.
From experience, In some versions of AOS the GUI displays extended SSID as disabled but it isn’t. If extended SSID is enabled then the mesh link will fail.
Check from the cli
and if extended-ssid is shown as enabled use the cli command
When the config is sychronised remove the ethernet cable from the IAP being used for the far end (point) and reboot.
All IAP's should be running same code version