Feeding my connection from router to better router

So I am reasonably techie but not so much in the networking dept. I would prefer to get some advice from a pro. I have two xbox ones that I would like to have both with a permanent open NAT type. Unfortunately this can only be done through a router that allow upnp and the 'Pace 5268AC FXN' supplied by AT&T absolutely sucks and doesn't allow this. The DNZ and just all around router settings are awful.

So I am going to buy a good d-link router that allows more functionality in the settings, specifically upnp and easier port forwarding. Can i switch it out for the default router my ISP provided or do I have to a 'feed' the connection from the gate router to the new router in order to be able do what I'm trying to do?

Any advice would be appreciated and any info on peripherals I also might need to do this would be great.

Thanks!