For APs to maintain Mesh, they must see each other at >= 25 RSSI.
You can review the AP's support info, for client details which will show the other AP as a client.
Mesh AP have very low RSSI value to establish a mesh connection to the root AP. The optimum value suggested to have is 30 - 35 RSSI.
### MESH Uplink History ###
BSSID S LastSeen Ch AdvUpl SmpUpl CalcUpl RSSI/UL/NF Flt IF UR D Seen Management-MAC SSID IP
c0:8a:de:a3:1e:c3 U 77079 11 130.00 12.00 12.00 19/13/-105 u - u 1 12 c0:8a:de:23:1e:c0 Mesh-101103000023 192.168.64.78
c0:8a:de:a3:1e:c3 U 77045 11 130.00 12.00 12.00 19/13/-105 u - u 1 48 c0:8a:de:23:1e:c0 Mesh-101103000023 192.168.64.78
c0:8a:de:a3:1e:c3 U 75446 11 130.00 18.00 18.00 20/14/-105 u - u 1 48 c0:8a:de:23:1e:c0 Mesh-101103000023 192.168.64.78