Now that lockdown is being lifted and I drive more, I've spent the last week trying to solve this, as I don't have WiFi on the road. After 4 trips to T-Mobile, several calls to T-Mobile and Samsung, and two trips to UBreakIFix for Samsung service on the device, I came to a sort-of solution, which is hopefully temporary.
Sometime back an update got pushed that wiped out mobile data.
My SIM works fine in my old Galaxy S7, but not the Galaxy S20+
T-Mobile changed SIM twice, verified account has no issues, verified settings looked good, and then pretty much gave up and I resorted to backing up my stuff and doing a factory reset, which STILL didn't get back the cell data.
So I went to the Samsung service shop, and they flashed it back to the original firmware.
Magically 5G works beautifully, no issues with data.
And then 30mins later, T-Mobile starts pushing unskippable updates to the device.
I went thru them, checking my data inbetween each update.
The 3rd update was "One UI 3.0 (Android 11)" and after that update my mobile data was gone.
I let it apply the last update, and still no data
So I started digging and found reference to this issue BACK IN JANUARY! It even has a sortof workaround, if you change mode to 2G (specifically) and slowly work back up to 4G/LTE, you'll get 4G/LTE data working. If you change it to 5G, it'll be broken again (and you can repeat that process to get back 4G/LTE data)
However, I paid an awful lot for this device, and I chose this model specifically to get 5G data when I'm driving, if I wanted 4G I could keep using my old S7. So I went back to the Samsung shop and had them flash it a second time. This time during setup, I removed the SIM and skipped everything so I could go into settings -> software updates and turn off Update download. I also went into Developer Settings and turned off the auto-update on restart option. I then set everything else up, and so far so good.
Now the problem is... I have a phone stuck in limbo, no security fixes and I don't get to enjoy the new OS features of Android 11
WHEN WILL THIS DEFECT BE FIXED??
Either release a patch, or we need to get mobile operators to stop pushing the broken update.
Each T-Mobile tech checked the APN settings, I didn't eyeball all the details myself, but saw them going in and scrolling thru them to check. Maybe they missed something but it went through like 5 different people in the store, plus back and forth with the telephone support escalation folks.