Ok guys thanks to
@TEKHD this issue has been resolved. I cannot thank him as well as everyone else who has tried to help on this issue. I spent $800 and was basically freaking out thinking I had bricked my phone. Thank you so much!
So my problem was related to the mps_code.dat file in the /efs/imei/ directory. After attempting to install Detonator for T-mobile..............this file was altered to read TMB.
When I tried to reflash with the correct stock ROM (model SM-N960F/DS region MYM N960FXXU2ARJ1) it didn't quite take.......and I'll tell you something I don't think I mentioned before. After the flash when setting up the phone I noticed I only had to agree to 4 things - terms of service, privacy, etc.) but I remembered that when I initially set up the phone the very first time, I had to agree to FIVE things.
To remedy this problem the solution turned out to be rather simple (of course) - replace TMB with MYM and then go into download mode, reflash the correct stock rom. This, of course, required root access. I used ES FIle Manager and followed Dr. Ketans latest version of rooting the Note 9 (including allowing modifications instead of read-only in TWRP). I installed his kernel and the oem patch just to be on the safe side.
After reflashing the stock rom I inserted the sim card and it worked perfectly as it did the first time out of the box. However, I still didn't have wifi calling so I decided to install detonator.
First, however, I backed up my EFS dir with Dr. Ketan's EFS backup too. I also backed it up in TWRP just in case.
Then I followed Dr. Ketan's procedure for rooting the device again.........and proceeded to install Detonator by booting into Download Mode, installing TWRP via Odin, then simply clicking INSTALL on TWRP and selecting the Detonator package.
Wifi calling is working 100% now as you can see from the screenshots.
Here are links to all the guides I used:
Dr.Ketan's How to Root a Note 9 - This will give you access to modify system files.
https://forum.xda-developers.com/galaxy-note-9/how-to/root-note-9-n960f-fd-experimental-t3832143
Tekhd's Detonator for T-mobile - this will mask your international Note 9 variant to work 100% with T-mobile (wifi calling, etc.)
http://tekhd.com/detonator/#tmodetonator
Samfirm - this is the tool where you punch in your exact model and region code and it will find a stock ROM for it. Super useful, free, and super fast download speed.
https://samfirm.en.lo4d.com/download
If you follow those two guides to the letter and have purchased an international N960F (or F/DS) phone and have T-mobile, you will get Wifi calling to work!
Thanks a million again to everyone who has helped. Seriously I am floored by everyone's kindness and generosity. Thank you so much.
P.S. As you can after installing the Detonator the codes in Service Provider SW Ver. gets changed to TMB/TMB/MYM.
Having never rooted an android phone before, I will say that I have learned many lessons by fire and if anyone has a similar problem please send me a PM and I'll try to help as best I can.