DNS Rebind Protection Detected

There doesn’t seem to be much info on this other than the support page that loads up when a plex client detects a rebinding issue:
https://support.plex.tv/hc/en-us/articles/206225077-How-to-Use-Secure-Server-Connections

I bought a surface pro windows machine and downloaded the windows store app version of plex.

I have an asus router running a merlin cfw and all the support page says is:

"Modem/Router Settings

We can’t provide information for all possibilities, but using ‘dnsmasq’ with DD-WRT or running pfSense are possible situations where you might run into this."

I can ssh into my router and there is a guide to explain how to create a dnsmasq config file:

https://support.plex.tv/hc/en-us/articles/206225077-How-to-Use-Secure-Server-Connections

examples in that article are address=/test.com/127.0.0.1, but that doesn’t match up with Plex’s suggestion of rebind-domain-ok=/plex.direct/

Am i barking up the wrong tree here? Has anyone managed/needed to do this for an asus router?