Just to follow-up on this situation. I contacted both Teamviewer and Tmobile. The application logs state that Tmobile is blocking port 5938 which was confirmed by Teamviewer support. I passed this information along to Tmobile and they escalated it with a ticket. However I receive a voicemail 2 days ago stating they would not be unblocking the port with no explanation as to why it was blocked in the first place. I don't know how many people on Tmobile use Teamviewer but unless a large number of people start screaming, the app will become almost useless.
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Julia, This issue is still ongoing. I just got off the phone with T-mobile support and they have confirmed that *.Teamviewer.com is whitelisted. The issue first appeared for me when I got my new Galaxy S8 so I thought it was a device issue. However when I saw that it was happening for both Android and IOS, I knew this was something bigger. Here are the troubleshooting steps I've taken: Uninstalled/reinstalled the application Reviewed and granted the teamviewer app all permissions it can possibly have Disabled/Enabled 4G/LTE Verified that I can access teamviewer.com and master12.teamviewer.com within my S8 browser. Lastly I emailed a copy of the event log from my phone to firstname.lastname@example.org. Please have your developers review this. The app keeps stating to check the Internet connection but the teamviewer app is the only application I have installed that can't seem to detect the mobile network.
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