How to set MTU 1508 on WAN (DSA)

I know that the isp supports frames of 1508 bytes, but I was never able to make it work in 19.07. Now I am trying to figure out how to do it in 20.02. I am using PPPoE with VLAN tagging and if I set mtu 1508 on wan, then it only changes MTU on eth0 (which is lan), but the wan interface remains 1500 and pppoe-wan is still at 1492. Anyone knows how to set mtu on wan to 1508?
The router is WRT32ACM with DSA.