@Taymay,

There are a couple of things that can influence the NAT type for video game consoles, however, it is recommended to search for a solution on the Playstation or Xbox forums, as they will detail specific ports that may need to be opened in advanced settings.

The First thing that I would check is to make sure that you only have one router in your network configuration. If your T-Mobile Gateway, modem/router combo, is still broadcasting a network or has not been placed into bridge mode, you will receive a strict NAT. To fix this, either place the T-Mobile router into bridge mode to eliminate the router functionality on the device, or place your Archer into AP mode.

The Other setting that I would want to make sure is enabled is UPnP as this is what allows consoles to connect using different ports.

As I said, there are a lot of things that can affect the NAT type; even having another video game console on the same network can cause the network settings to be restricted.