PPPoE adds another 6 bytes of overhead and the PPP field adds two more bytes, leaving 1492 bytes for IP datagram. Therefore max PPPoE MRU and MTU values must not be larger than 1492. TCP stacks try to avoid fragmentation, so they use an MSS (Maximum Segment Size).
Apr 17, 2018 RFC 4638 - Accommodating a Maximum Transit Unit/Maximum RFC 4638 PPPoE MRU/MTU Increase September 2006 An optional PPPoE tag, "PPP-Max-Payload", allows a PPPoE client to override this default behavior by providing a maximum size for the PPP payload it can support in both the sending and receiving directions. When such a tag is received by the PPPoE server, the server MAY allow the negotiation of an MRU larger than 1492 and the use of an MTU larger QUAL O MELHOR MTU PARA PPPOE? - YouTube Jul 22, 2018 Juniper SRX - Configuring PPPoE
Windows machines from visa on will auto determine the MTU and would match the router setting. @Oldjim For what it's worth: Plusnet TG582n router will by default use a MTU of 1492 when configured for pppoe (fibre). When configured for ADSL it defaults to a MTU of 1500. Don't know about the 2704n.
MTU/MRU When a PPPoE-speaking DSL modem sends or receives Ethernet frames containing PPP + PPPoE payload across the Ethernet link to a router (or PPPoE-speaking single PC), PPP + PPPoE contributes an additional overhead of 8 bytes = 2 (PPP) + 6 (PPPoE) included within the payload of each Ethernet frame.
MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). The default MTU size is 1500, however for some networking technologies reducing the MTU size and allowing fragmentation can help eliminate some connectivity problems occurring at the protocol level. RESOLUTION:
How to Set Up PPPoE Internet Access - Lifewire May 09, 2020 Understanding MTU and MRU Configuration for PPP