We had/have a similar (or maybe the exact same) problem (still have it but much more rare than it use to be) in that the client radio would just stop trying to connect after a few tries if it wasn’t getting a response from the PPPoE server.
They (cambium support) wanted packet captures from between the AP and the Switch… which was going to be a huge pain.
What I ended up doing was using the firewall / logging feature on the CPE and AP to log PPPoE Discovery Still having PPPoE Problems I was able to show them that the Client radio would make a couple of attempts to auth and then just stop trying. Like you , rebooting the AP (or maybe not like you, just kicking the radio off the AP or anything that caused the radio to have to reconnect to the AP) would usually cause the customer radio to start trying again and work.
If you do end up using the firewall to log PPPoE I could not get it working until (at least with the firmware available at the time) until I went to Configuration > System > System Logging (Syslog) and enabled all under “SysLog Mask” (just creating the firewall rule and setting it Log did not work). Not sure which one of them makes it work or if that step is still required as I’ve never taken the time to test it.