Virtual Private Networking (VPN) is a secure (encrypted) way of connecting to private resources (business LAN, home computer) over the public Internet. To make a VPN connection, VPN client software typically connects to VPN server software. VPN types include Point To Point Tunneling Protocol (PPTP, deprecated but still useful) and OpenVPN (recommended, secure). But you may have a problem making VPN connections over T-Mobile USA wireless data.

The Symptoms: You are able to do most things on your mobile device over T-Mobile data (browse the Internet, send and receive email, etc), and you are able to make VPN connections over Wi-Fi wireless, but you are not able to make VPN connections over T-Mobile wireless data.

The Cause: T-Mobile apparently pushed out a data configuration in late 2013 that changed APN Protocol to IPv6. That works properly for most things on mobile devices, but it prevents PPTP (built into Android) and OpenVPN (OpenVPN Connect) clients from making VPN connections.

The Cure: Open Settings > Wireless & Networks > More... > Mobile networks > Access Point Names > T-Mobile GPRS (fast.t-mobile.com) > APN protocol, and change the selection from IPv6 to IPv4/IPv6.

The Caveat: This cure was tested successfully with a Nexus 5 on a legacy FlexPlay plan, and it might not work properly on other devices or plans. Use at your own risk.

Source: Navas' Tips