Not able add TS350SE with zero touch Mesh

[ValmetNL]

Hi,

We have two TS350SE access points and one is located outside in a cabinet (AP-1) and hard to connect to the other one by cable. I gave the outside unit a factory reset (firmware version is the same as the local unit) and added it's serial number to zero touch mesh in the Root AP. I can see in the zero touch mesh screen that it status is joined, but the access point is seen as disconnected in the Access Points screen, and the system log tells:
Root AP accepts MESH connection
Connection request from AP-1 is refused, Unleashed does not support discovery across subnets.

Does anybody know how we can resolve this?