My ISP also instructs me to set the MTU of the PPPoE interface to 1508 to prevent MTU problems.

See also https://community.tp-link.com/en/business/forum/topic/257626

I disagree @Hank21 : Per definition a switch forwards packets at network layer 2: ethernet. A router forwards packets at network layer 3: IP. NAT is not what makes a router a "router". A normal...

After upgrade of oc200 to 4.4.4 I see that prefix delegation is supported when dynamic configuration with SLAAC/DHCP is selected. Yay ! It works !

@Somnus Indeed, I drew my conclusion from the file name. But after installation is does report as 4.4.4. Thanks !

Yes, unfortunately it doesn't support DHCP6 prefix delegation that providers in the Netherlands use to assign prefixes (mostly a /48 or /56). Without that support I still can't use it.