The first most important item to remember is that PPPoE IP Frames are limited to 1492 Bytes.
OpenWRT claims that the "MSS (Maximum Segment Size) Clamping" Feature (option mtu_fix '1' in the /etc/config/firewall file) takes care of this (it even gets added automatically when configuring a PPPoE WAN Interface), but in reality it is safer to manually configure it: option mtu 'WhatEverValue' (1492 in the case of PPPoE) in the /etc/config/network file.
https://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet