Basically the problem is that on a T-Mobile iPhone, certain apps absolutely refuse to work when connected to the cellular data network, even though the phone is set to allow all apps to use cellular data. The apps load just fine, but when they try to do anything requiring network access, it's as if it's not even on the network, when the phone clearly is on the cellular data network as evidenced by the fact that other apps work with no issues!
Most notably, one of the apps experiencing this annoying issue is called iTeamtalk, a high quality voice chat app. It refuses to connect to any teamtalk server when on Cell data. However, this is not the only app I use that's experiencing this problem. I've discovered at least 3 apps that will not work via T-Mobile cell data. In all cases, the apps act as though they are totally offline and will not connect to anything. For example, the Magic Jack app, an app that allows one to make calls to telephones, will not connect to the magic jack server, and thus cannot register itself to allow the user to place a phone call.
Transmit is another app that won't work via T-Mobile data connection. Transmit is a file transfer client that supports FTP and SFTP file transfers. When trying to connect to any server, be it an FTP or SFTP server, transmit immediately throws an error stating connection timed out or server hung up. It's as though it's not even trying to connect at all. Even stranger still, another app that can do file transfer via SFTP and FTP among others, is file browser, and that app can connect to anything I tell it to when on T-Mobile cellular data without issue! And as if that wasn't strange enough, it gets even weirder! If I enable personal hotspot on my T-Mobile Device, then connect to my T-Mobile hotspot with my Verizon iPhone 7 plus, which has the exact same apps on it, I can use all the apps such as transmit and iTeamtalk via the T-Mobile hotspot with no issues whatsoever! I can also use these apps when directly connected to Verizon's cell data network without any issues of any kind. I want to know what's causing these 3 apps, and possibly more, to simply not work if being used directly on a T-Mobile iPhone on the T-Mobile data network? It makes no sense at all. Why do these apps not work, but others work without problem? When I use cell data, I expect every app I have to not have an issue using cellular data!
This is quite an annoying issue, and since I cannot diagnose the problem any further, I'm unsure if the blame is with T-Mobile or apple. I do know that I'm not the only user experiencing issues with apps refusing to connect to the internet when on T-Mobile data directly! At least 2 other friends of mine in totally different areas of the country are experiencing the same issues as me, and the only common factor is T-Mobile. I'd be curious to know what other apps may not work via T-Mobile directly. The fact that the issue was briefly fixed, then broke again is rather curious as well. Anyone that has thoughts or comments on this I would welcome them.