I finally found a solution that worked for me. It was in a T-mobile forum. Here's a link to the article:
https://community.t-mobile.com/troubleshooting-38/globalprotect-vpn-not-working-with-t-mobile-35992
In case the article is archived, here are the instructions:
Also confirmed reducing MTU value in netsh configuration resolved GlobalProtect VPN issue for Windows 10.
NOTE: You need root/admin access to be able to change it through command line.
1. While connected to VPN, open a Command Prompt CMD (Right Click CMD -> Run Ad Administrator)
2. To verify your Virtual Ethernet Adapter Interface, Type the following:
netsh interface ipv4 show subinterfaces Change Windows MTU Size
3. As mentioned on the above replies. credit Rich T, Type the following:
netsh int ipv4 set subinterface “Ethernet 2” mtu=1300 store=persistent
Replace “Ethernet 2” (keep the quotes) with whatever name your computer uses for the globalprotect virtual interface in ipconfig.
4. Test your sites
Browse some Corp and Internet sites while connected to the VPN.
Note: You may have to restart your VPN connection by disconnecting and reconnecting.
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