RT-AC86U and dual wan

As wired internet is so rubbish here, I've gone the way of a 4g router to compliment my VDSL line. I had them in a dual wan load balance setup, but this made quite a few sites and services a bit unhappy so I've dropped it back to a fail over setup.

Ideally what I'd like now is to have a default WAN port on the router, but allow specific devices to use the secondary WAN. Is there any way to achieve this without going to back to load balancing?

I'm using an RT-AC86U and the latest merlin...

RT-AC86U and dual wan