to your build.prop and finally using adb from a PC (u can try this even if not rooted) type the following from cmd or powershell: "adb shell" followed by "settings put global tether_dun_required 0" without the " "s and restart your phone n hopefully that should do the trick.
If still throttled by TMo go into Magisk and install tethering_enabler or TTL tethering modules. These modules weren't specifically made for the 6T but they should still be effective. If still unsuccessful then modifying the TTL from the device your trying to tether to will help hide the source of data aim for TTL of 65 to confuse tmo.
If your not rooted then use PDANet USB tether to a PC and hide tether options then create a hotspot from the PC your USB tethered to or use the WiFi Sharing option in PDANet PC Menu. At least thats what works the best for me and I've tried many many things... One other possibility if your on stock no root is to add a new apn with all the same info as normal fast.tmobile.net lte apn only enable MVNO type to GIO for apn type leave it blank and save it. This should hide the default TMobile apn (and make the editor think your using a 3rd party like Mint) now go back and edit the new apn and under apn type put: dun,default,mms,supl try and save it. It should error n say dun not supported... Press Home and force close settings-reboot phone. The dun sometimes will stick if this method isn't patched yet I'm sure it will be soon.
Your final options are to use an app from playstore like VPN Hotspot or VPN Tether they might slow your speed a bit but they should tunnel all your data thru a VPN confusing TMo as to the origin of data. Probably the best and easiest way is by using a custom ROM as most haven't been tainted by Google's latest CarrierConfigManager commits or carrier apn restrictions. If anyone knows how to reverse these commits easily or any other methods not listed please let me know.
I'm currently looking for a clean TMo only dun enabled apns-config.xml so I can overwrite the default n modify the build.prop for a quick fix on any new ROM or update...
As you can probably tell being throttled or having to pay 2x for the same data is a pet peeve of mine and I've spent some time researching the topic. I'd love to hear what ended up working for you or if anyone knows a solid method I missed. Or has a recommendation for a good ROM with unlimited hotspot features built in for the 6t let me know. If any of these Bypasses helped you hit the thumbs up so I don't feel like I wasted hours of my life making this guide