(SORRY SO LONG) I really don't have any solution, only an explanation as to why I don't believe Tmobile holds the entire blame, if any at all.
SIDENOTE: Whoever the person is ... posting they're going to swap out their iPhone:
(1) It's not a hardware issue, and DEFINITELY not your iPhone.
(2) Even if you don't believe anyone - good luck with trying to swap your iPhone either Apple direct or with an Apple Store. You won't get anywhere.
Just understand, know and be resolved in knowing it's NOT a hardware issue. Your iPhone is FINE.
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I don't quite believe this is a T-Mobile issue either - as they've been outright blamed here. We've had either at one time or another, one to several different versions and models of d-link cameras, and using on different versions of d-link's lite app succesfully. We've also been with T-mobile, basically ever since they started up in the states, NINETEEN years.
This error number has historically reared it's head several times over the years, and it's ANNOYING to see it when it does because it's always been on d-link's side, and they seem ALWAYS slow to move to correct it whenever people start reporting it. The only good thing about it is (if any) there's enough wifi /hotspots around thta you can connect to - you can get around the problem.
"Getting around" the problem, or doing a "work around," however, should NOT be required! Granted - I'm totally grateful to what D-link DOES provide. For example: I can set up email and video /pic file notifications for I believe pretty much all our cameras. Foscam, for example - you get to do the same process for ONLY ONE camera. How lame. They aren't even storing your image(s). BUT... when D-link already is aware of this type of issue, they are extremely slow to address the issue, and worse to deny the issue, and not provide any viable short term solution.
BACK TO TMOBILE:
Here's the deal - D-link has historically had issues with it's updates and this error. Their App version history shows they've had FOUR updates just this year alone. However, this error has NOT been a constant with Tmobile, and as mentioned, I can attest to being with Tmobile for 19 years.
D-link recently had issues with the last couple versions of their app. With the last version, maybe two ago - the app wouldn't even give an error. It simply CRASHED every time I tried using cellular. After updating the app - without crashes, I was able to connect via cellular again, just fine with Tmobile.
Before - with this error I had been told (or I read) to disconnect and reconnect the cameras displaying the error.
OH YEA... because that's the "OTHER THING". Not until this last bout, we would not get this error for all our cameras. Only one or some would display the error, and once we reset the power - the error was gone. However, it seems (as I've tried) this is not an option this time.
All this to say - maybe it is the way Tmobile delivers it's data (CDN) at certain times, and probably not. But we've had no connectivity issues for the last few months, since the app was updated (partially for crashes, maybe) But using Tmobile in these last few months , we've had successful operation, so I don't know how Tmobile could be to blame in totality? It's gotta be in the app.
Someone replied early on here reporting the POST #5 was a successful option. And if you don't have VPN like most of us don't - it might work for you. I've not had a chance to try it, but someone right after the post - reported success. You'll just want to make note of your initial settings so you can change back to your original settings.
The post noting something was - "in the works" (referred to via a link in the last post) was made nearly three weeks ago. So lets hope D-link gets a REAL FIX SOONER than later. Unfortunately, D-link knows the issue... but always slow to move.