You don’t need to change/increase the MTU setting of the ethernet and VLAN interface underneath the PPPoE client interface. You only need to set Max MRU and Max MTU to 1500, and make sure that L2MTU, if available, is at least 1508 (the value is 1510 in my screenshot below). However, it’s recommended to create a separate PPP profile with Change TCP MSS = no in that case, and use that profile for the PPPoE client. Otherwise RouterOS will wrongly reduce the MSS value of the TCP connections.
