Grandstream GWN7000 DNS rebinding affecting PLEX

Ok, so I have my new grandstream router up and working, but I'm having a problem with my PLEX server. I think it is to do with DNS rebinding as per here, as I can no longer direct connect to it securely. I had the same issue with the fritzbox but it was easily fixed by adding plex.direct to a box and it fixed it. From what I can find the way to change it on the gwn7000 is to go here

and add a line to the DHCP options - rebind-domain-ok=/plex.direct/

but that just breaks the internet.

Anybody got any suggestions here? I don't even know what DNS rebinding is, so I'm a little out of my depth.