I have a couple of A5’s with a couple dozen C5’s connected to them, and I have seen this issue 4 times in the recent weeks, and probably a dozen times in the past 6 months.
For legacy reasons I use PPPOE on the client side so in this case the client’s router is PPPOE client through the C5 to the Mikrotik NAS sitting behind a Netnoix switch which is tagging/untagging VLANs and powering the A5 at the tower site.
The A5 is on one VLAN for management (100) and there is another VLAN trunked through the A5 to the C5 (101) for CPE management.
What is happening is that randomly a C5 remains manageable via the wireless interface but it won’t pass PPPOE and it cannot be managed over the Ethernet interface using 169.254.200.20. The Ethernet Interface is up and not flapping.
When the C5 is taken out of service and brought back to the office where it’s not in range of the A5 it can then be managed over the Ethernet interface on the 169 IP (directly plugged into the same POE and Laptop that it wouldn’t talk to when deployed) and after performing a Reset Defaults it can be re-programmed and successfully redeployed. I’ve only just tried this on the 2 most recent cases and they are both back in production now - I wrote the others off as borked
The C5’s are properly grounded and the issue randomly affects different C5’s. Rebooting the A5 does not resolve the problem. Replacing the POE does not resolve the problem. All are running the current firmware.