WISP AP MODE bridge use

PPPoE client devices connected to the PPPoE server falls off after 2-3 hours.
I checked wireless connected to the AP but also of PPPoE is broken and can not connect.

I have re-configured the device did not improve.
When the device reboot of the client is connecting to drop again after 2-3 hours ...

WHAT'S THE PROBLEM?
SAR2 SXT devices do not this problem.
I am not happy SXT from the AC series !!!

PPPoE client disconnect from the server at the time of log records.

PPPoE client disconnect from the server at the time of log records.

There are a foreign mac address I wonder if it may be involved in

Where I find replacement parts????