Does yours work in areas with only Band 12?
I can verify this fix works!
I had a Samsung S7 Active unlocked from AT&T with this same exact issue. I called T-Mobile support and they had me go through all the debug steps - New APN entry, clearing cache on my browser, clearing storage on multiple apps on the phone. None of it worked. I stumbled across a work around on my own - turn airplane mode on, wait a few seconds, and then turn it off. Data would come back every time. But only until the next reboot when I would lose it again. These steps provided here in the OP work as a permanent fix. Data is on every time after a reboot. Thanks for posting the fix! I only wish that the T-Mobile tech had thought of this.
Before I deleted all the existing APNs, I noted that there were about a dozen of them on my phone. My theory is that the S7 is not defaulting to the correct APN for the T-Mobile network - even after we add a new one and select it as the new default. So on reboot, another one of the APN configs is probably being used from cache, and that's why we are seeing this issue. The new default config created by following the T-Mobile knowledge base article in the last comment is loaded when turning data off and back on again with airplane mode but for some reason is not loaded with a reboot.
When we delete ALL the APN configs save the one, correct config for t-Mobile, then the phone has no option but to load that config at reboot.
That's my two cent theory and I'm sticking to it!
On the APN Type make sure is not set
APN type: not set
I had put in Internet+MMS and my data was still not working, I delete it and now its set on to not set. This work for me, also following what galaxyowner7 said.
Hope this helped as well!