At this time, some T-Mobile users may experience issues connecting to the Internet when using the Onward VPN on the T-Mobile data network. You shouldn't have these same problems when connected via Wi-Fi.
T-Mobile uses IPv6 technology which effects many VPNs (Virtual Private Network), including ours. We are working to resolve this issue and hope to have it resolved in the coming weeks (February 2018). Our next release (2.9) may support T-Mobile. Sorry for the trouble in the meantime.