Hey there! What kind of phone are you using?
You should be able to put your device in IPv4/IPv6 mode -- or IPv4 only mode in order to do this.
Some folks have said services like VPN aren't working because they're getting an IPv6 address on their device and the service only supports IPv4. Their solution has been the dual mode or IPv4 mode, depending on your preference.
Manage your T-Mobile account