5th Attempt: I'm getting tired of trying to post my question. Maybe this one will work. Problem: Mobile Hotspot didn't work, T-Mobile Tech Rep ruled out all cell-data-signal problems as Network appeared & we did a few things & nothing worked. He sent me to Microsoft, & we talked for hours today, then she said my case would be escalated and I'd get a call in 11 days. Googled problem on the phone & it directed me to APN settings? The guy said the bottom should not be IPv6 only, and it was on my phone, and I couldn't change it, T-Mobile GPRS, the only APN, & so the guy (in some Tech mag) instructed me to make new APNs. One for Internet Everywhere and the other for MMS eeservice. Voila! The Hotspot worked, but it seems weird, doesn't it? Is it risky? How can these random APN numbers all of a sudden work while T-Mobile and Microsoft spend hours with me and we all can't solve the problem. If the Tech Reps don't inquire about APN settings, then how does the change in the APN settings make it work? Plus, they're just random APNs? I don't even know what they are. They're just numbers to me. One correspondence is that now my location is DALLAS like the APN runs me through there or something though I live halfway across the country. That made me suspicious enough to inquire here. Do you guys have any thoughts? T-Mobile screeners welcome. If three heads can't figure it out then maybe 1,000 can. Are there any risks to the kind of solution I found online? Nobody asked about APN numbers, the Tech Reps I mean, so I was surprised it made the internet work. Should I accept this as a fine solution since the Hotspot connection on the computer works now? What are the risks of the random APN numbers. I remember when the T-Mobile guy asked about Mobile Networks & T-Mobile GPRS, and I told him it was checked, he said, "Oh Good!" like that was a good thing, but then it didn't work, and the problem was in there with the information in the boxes? I don't know. Can anybody help or should I trust this weird Dallas Connection half-way across the country?
APN is, for lack of a better analogy, your cell phone's "cablemodem connection". It tells your phone what to look for and how to connect to get to the internet. I think it's something to do with how to talk to the towers. Or, possibly the "gateway" information on how to get out to the Internet.
My guess as to why IPv6 didn't work probably has something to do with the device you connected to Hotspot. IPv6 is a new standard that's taking quite some time to roll out and most people / companies aren't enabling it on devices by default. IPv4 is the addresses people are "accustomed" to seeing (like your home router might be 192.168.1.1). IPv6 is a lot longer in hex (like 2001:0:903d:3ba9:8304:1099:9f32:ab33 [which is probably not valid, for people taking notes -- it's just a representation of what I see on my laptop]). A few years ago, virtually no ISP worked with IPv6. More and more are now. They're running "dual mode" where you can get an address of each, because some internet sites / apps don't work with IPv6 yet (VPN has been a large complaint in the past, maybe it's mostly resolved?). My experience with T-Mobile on my phone is that IPv6 is faster. This was a couple years ago when it was first rolling out, and it potentially could have been the IPv6 network had very little traffic. With the additional rollouts of IPv6 on phones, it's probably gotten a tad slower.
The good news is you don't need to worry about security. You cannot connect to another APN other than T-Mobile's. Your traffic is originating within T-Mobile's network, so it needs the T-Mobile address (APN) to go out in the world. Similarly, if you're at work, you cannot give your computer your home router / cablemodem information to access things. Your connection information is associated to the network you're connected to. You're connected to T-Mobile's network, so your APN isn't hijacked.
Why does it say you're connecting through Dallas? The APNs try to assign you to a "exit point" that's fairly close to you, but sometimes there's no availability, so you get sent somewhere else for a while. This question has been asked a few years ago, and that was the answer. Every so often, there's a "rebalance" where you may get a different "exit point" that's closer. With most standard browsing, you should not see anything noticeable if your "exit point" is Dallas or Boston or Seattle [or whatever "exit points" exist]. You would probably notice if you were online gaming where lag is a concern (Pokemon Go gym battles / raids for example - if you're in a group, you'll see some people's battle starts a second or three before others - this is probably because of syncing that is related to something like this lag).
As for why support didn't ask? That's a decent question. It's probably because most phones are set for dual IPv4/IPv6. And, most hotspot clients are now supporting IPv6 (I was surprised to see an IPv6 address to show up on my laptop when I upgraded to Google Wifi for my router -- until I remembered I had IPv6 set up on my laptop). That means folks should be able to assume this is already on, so it doesn't come up. You don't say what phone you're using as hotspot nor what you're using to connect to hotspot. It sounds to me like what you're connecting doesn't have IPv6 enabled (which means it's potentially a couple years old). Most support is taught to troubleshoot "this is what the world looks like now" -- as inconvenient as it is. Otherwise, you have millions of options available to train on and a support agent may only use a few dozen of those. There SHOULD be a knowledge base they should use, but I've learned first hand from the folks who man our helpdesk where I work that they're generally not helpful because Person A wrote the knowledge base article and used terminology that's not used in the keywords Person B is searching for ("Hotspot partially incompatible with IPv6 Only APN" won't be a high search result if someone searches for "Hotspot not working" -- and IPv6 probably wasn't talked about in your support calls to try to narrow it down or flag it).
Support isn't a perfect option. Your idea of 1000 heads on your problem instead of 3 is actually a better support option, in my opinion.