Oh cgarcusm, that's a bummer this message is ruining your gaming experience.
This type of issue is a little outside of our scope of support here on the forum. We can help with the initial setup of the router and troubleshoot connecting to it with other devices. After doing a quick Google search of that message, it seems like a lot of folks have several different ways of trying to fix this and some have to do with changing setting within the console. You may want to try running this by XBox support team to see if they have any ideas.
Hey cgarcusm! Just wanted to check in and see if Mike's suggestion had helped - did you reach out to XBox support? If you have any useful tips in case another forum user has the same issue in the future, we'd love to know how you solved your issue! Thanks, hope everything is going well.
Do not put your console in DMZ. That is opening it straight to the internet.
What firmware version are you running? It seems like firmware 126.96.36.199.376.3169 has crippled the router. You are unable to PortForward or utilize UPnP.
Unofficially you can try and flash the retail version of this routers firmware. This will open your NAT for the xbox and allow Portforwarding etc.
T-mobile needs to ask ASUS to push a firmware update to restore the routers capabilities.