From https://forum.vyos.io/t/missing-mru-in-dial-pppoe-interfaces/11880/4?u=c-po
https://linux.die.net/man/8/pppd
mru n Set the MRU [Maximum Receive Unit] value to n. Pppd will ask the peer to send packets of no more than n bytes. The value of n must be between 128 and 16384; the default is 1500. A value of 296 works well on very slow links (40 bytes for TCP/IP header + 256 bytes of data). Note that for the IPv6 protocol, the MRU must be at least 1280.