Clients connect to mesh AP without internet access

I have this kind of setup:

There is a multi mode fiber coming to my property. It connects to a router. From the router it connects to a switch, and from the switch it connects to four separate buildings. In each building there is a media converter, and from the converter it continues through Ethernet to access points, one in each of the four buildings. The buildings are semi-detached houses. On the other apartment of each house, there is an access point extending the wifi by mesh technology.

All access points are NWA50AX, and all the AP:s make one wlan with same password. They are in same plan in Nebula.

All AP connecting through Ethernet are working properly, but a couple of the meshing AP:s allow clients to connect to wlan, but the state of the wlan says "No internet access". And to make it even stranger, after reset they work properly for a while, but eventually internet access will cut off (connection to wlan remains stable though). If the mesh AP:s are plugged off, the next door signal will come through the wall and client devices will gain access to internet again, but signal is not strong enough to reach all rooms. So problem seems to be definitely with the mesh AP:s. I have also tried switching AP:s, but always the mesh end fails.

Main router is in DHCP mode, and access points are in NAT mode. The main router has a limited IP pool, but it has more than half of the available IP:s free.

What could cause this?

I will gladly post additional info about settings if needed.

Attached a plan of the network, if it helps.