I'm switching from Xfinity and still have both network but my windows 11 laptop says can't use T-Mobile WiFi

Did you disable DHCP on the Xfiniity modem/router? The Arcadyan KVD21 5G Gateway from T-Mobile is a DHCP server and it can't be disabled. There can only be one DHCP server on a network.

As an experiment, power down the Xfinity device, reboot the T-Mobile gateway and your laptop, and try connecting again. This advice presumes that you've already successfully set up the T-Mobile gateway. If you want to stick with the same WiFi name (SSID), set it up that way in the app. Alternatively, give it a new name with the T-Mobile app and then choose that WiFi network on your Windows machine.

I'll give it a try later. Thanks for the suggestion. I did give it a different name so I can easily tell which network was connecting . This is the first device that would not connect for me 

Assuming that your Windows machine is a relatively new one, it sounds like an IP conflict. If the above suggestion doesn't work, try creating another network on the T-Mobile gateway where the frequency band is 2.4 MHz and the WPA version is WPA/WPA2. This network would be in addition to the one(s) you've already created and which is successful with other devices.

If your Windows machine connects, your problem is most likely the network adapter in the laptop isn't able to navigate WPA2/WPA3 security. No problem, just use the network you created in addition to the others.

It's about 3 years old, I believe. 

Then you shouldn't have a problem with WPA2. If every other device is connecting to the gateway, there is no reason your laptop shouldn't be able to connect as well. Are you using a proxy, or a VPN, or have you changed your DNS settings? Turn off your proxy settings, if you have any, disable your VPN temporarily, and erase your DNS settings. DNS should default to 192.168.12.1. and be the sole entry. After your computer is connected to the T-Mobile gateway, you can modify DNS to, say, Google at 8.8.8.8.

One last thought: make sure you didn't assign a static IP to your laptop. That would definitely screw things up since it is almost a surety that the IP schema is different.

Only default settings, nothing fancy. There are no dns settings on the tmobile device. My Xfinity Google WiFi network has many settings work my home network. So i can change it to wpa2 only or wpa2/wpa1. But default is 2 and 3 wpa. 

Since it is a Windows client make sure there are no pending updates lurking in the wing. I find the Alienware machine on W10 can act strange when Microsoft has pushed out an update and it needs to have a reboot, which has not taken place. If the client is only a few years old I would guess it is at least an 802.11ac network adapter. To be sure you can confirm the adapter capability with the system information reporting. My guess is that it probably has to do with the authentication type. T-Mobile seems to have WPA2/WPA3 by default. A client that is only a few years old should have no issue with WPA2. 

To confirm the IP address you have with that adapter you can use the GUI via device manager or whatever it is called in W11 or use the command line. If you issue the command ipconfig /all | más you can then use the space bar to page down through the network information and see the details for the network adapter addressing. You could also issue the command ipconifg /release to have it let go of the ip address it has and then ipconfig /renew to have it request a new address via DHCP. The client will not request an IP address on the wireless network until after the authentication process to begin with. If the client joins the network but does not get an IP address I would agree it is possible a static IP address was assigned and needs to be changed for dynamic addressing.

Wpa2 only did not make a difference. The Xfinity connection is wpa2, but that network does support 3 as well. 

And there is yet another windows update, i thought that was the issue bit I had completed the available updates but there is always another one . 

If you are not sure what authentication type the adapter supports the best thing to do is to do a search online for the model of laptop you have and the specifications. If the vendor does not list the network adapter specifications then you might have to go find the user manual and pull it down. 

If you get on the 2.4 GHz frequency on the gateway and set the authentication type for WPA/WPA2 and then it connects that would suggest it might only support WPA and not WPA2. Knowing the age of an adapter helps but knowing the specifications and the authentication type is important. 

If the onboard adapter on that laptop does not want to play you can always buy a USB nano size wireless adapter that is 802.11ac or 802.11ax capable and that would probably solve the issue. I have had older clients that either did not have a wireless adapter or an older standard which did not want to work. Buying a newer 802.11ax USB wireless adapter would be a fix.

I'll look further as your suggest, but my Google WiFi has WPA 3 and it connects. So I really don't think that's it. I emailed Microsoft help. It's odd that it checks WiFi requirements for three network and then gives message it can't connect. I tried my Google WiFi guest network and got the same message.