How to set MTU 1508 on WAN (DSA) for PPPoE

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.

Works in 22.03 for me now (wrt3200acm). Sharing for anyone who wants to give it a try.

config device
        option name 'wan'
        option mtu '1508'

config interface 'wan'
        option proto 'pppoe'
        option device 'wan.X'
        option mtu '1508'
ifconfig pppoe-wan | grep MTU
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
ifconfig wan | grep MTU
          UP BROADCAST RUNNING MULTICAST  MTU:1508  Metric:1
ifconfig wan.X | grep MTU
          UP BROADCAST RUNNING MULTICAST  MTU:1508  Metric:1

Sweet. Could you name your ISP by chance?

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