Struggling with new rt-ac68u and bridged td-w8950n

Hi really hoping someone can help me with this as it has me tearing my hair out inf rustration

I am trying to use this with a tp link td-w8950n acting as a modem in bridge mode. I end up with a totally unstable connection and lots of modem hangup errors. one particular error is a page appears trying to connect to the router.asus.com with a dial which is trying to connect but over and over rapidly in a broken looking way.

I have tried with newest firmware and a couple of differnet merlin...

Struggling with new rt-ac68u and bridged td-w8950n