what values for dns / default gateway, etc do I use on asus rt-ac68u behind an att modem.

This is setup as port forward through passthrough and was done by the installation when I was asked if I wanted to let it resolve it.  Ultimately, I would like to use google's dns servers, but there are more locations than I understand.

I am not sure if the wan uses the external google dns address's, and the lan uses the internal att dns and gateway.  No matter what I do I don't seem to have dns set, and it times out.