RT AC56U PPPoA VPI/VCI

Hi, trying to connect the AC56U to a TP LINK 8817 and need to use Bridge Mode to avoid the dual NAT problem. The problem is the AC56U with suplied firmware does not support PPPoA and also does not seem to to be anywhere to put in the VPI/VCI. I guess the PPPoA issue is because it is not as common as PPPoE but not having VPI/VCI is essential.

I am in the UK so also keen to know if the Merlin firmware will work for my AC56U, I read something about some country specific models no working...

RT AC56U PPPoA VPI/VCI