This is how I got mine to work perfectly.
1) Make a copy of your stock Tmobile APN. You can call it MetroPCS or whatever but make sure every field is exactly the same other than the "Name".
2) In "APN type", put in " default, mms, supl, dun " of course leaving out the parentheses.
3) Go down to the "Mobile virtual network operator type" and change it to "GID". The "Mobile virtual network operator value" should automatically change to "6d38".
4) Select your newly created APN.
5) Restart your phone and confirm that the correct APN is still selected. It should now work. I have the exact same phone.
I have no idea why there is so much confusion about this and why no one from the MetroPCS tech support knew about this. After a couple hours on the phone with tech support, they came to the conclusion that I have the rare freak phone that would not tether. I ended up finding the solution right here on this forum.
I must thank bremos812 for that info. Good luck.