A router use pppoe to connect internet,it's wan port's MTU must less than 1492(when wan port's ethernet mtu is 1500,this is the default value).
In this situation,the tcp mss must change to "IP_MTU - 40", otherwise, a big packet may be droped.
Does anyone know that the action of adjusting the MSS is automatic or must be set manually?
If in the firewall settings "MSS clamping" is enabled on your WAN interface this should be handled automatically 