I guess it's something relating to PPPoE vs. PPPoA, as VDSL/ADSL use those respectively on BT - they're identical otherwise in connectivity. Do raise it with support dept. (or ask for progress or acknowledgement) if you're already reported it. Which ISP is this BTW ?

I doubt it is PPPoE vs PPPoA. This would not explain why it kind of work over WAN2 (using the OpenReach modem) and not over WAN1. My ISP is IDNet, if that helps...

Yes, fair point; I missed that bit..though it might still be related... but do chase support.