Use of 3rd party Wifi Router with my ATT FO Internet service

Sure.  You don't say which AT&T gateway you have and there are different methods depending on the gateway.

For the BGW210 and BGW320 gateways, use THIS procedure.

For the Pace 5268ac gateway use DMZ+ as outlined at the bottom of the page HERE.

@aapn 

Do you actually need it to work as a router or is the need more for replacing the WiFi of the gateway.   Most do this for the latter reason and in such cases it is much easier to run the personal device in access point mode.  Much easier to maintain the home network.   

Thanks. Realized I had forgotten to list the device after submitting & was planning to come back & edit. It is the Pace 5268AC.

Thanks again

Thanks. I came to the same conclusion shortly after I posted the question that I only needed a wifi access point & did not want to risk some of the security concerns by using it as a gateway.