Sprint / AT&T Galaxy S6 Edge UNLOCKED to T-Mobile Hangs & Never Dials a Number

Hi guys, I know that this is an old problem. But, I have a Galaxy S6 Edge SM-G925P unlocked with a T-Mobile SIM card. The phone shows LTE but when dialing a number, the phone hangs for about 30 seconds then the call drops. I immediately redial and the phone drops from LTE to "E" then the call finally goes through. When I finish the call the phone goes back to LTE and does the same thing if trying to make a phone call. Also, when I try to call the phone ot goes straight to voicemail. The soluting to this is to switch the network from LTE to GSM however I can only get 3G and 2G internet speeds.

I even flashed the phone with the G925P to G925F conversion but it does the same thing.

Also, a Galaxy S6 Edge AT&T SM-G925A which does the same thing. The only thing that fixes it is to downgrade the phone (If Possible) to Marshmallow 6.0.1 and deactivate / activate the LTE Enhancement on the settings (sometimes this does not work). With this phone, I can not change the LTE settings aince this is GSM only. Anyways is there a way that I can use the phone without having to downgrade to Marshmallow?

I know about the phones trying to use the T-Mobile VoLTE network. But please guys, I know that you have a fix for this. Please Hellpppppppp!