[Samsung Galaxy S21 Ultra] danblue00:
Thanks. frija is a tool that picks up the right firmware for download. So, I don't think there is any issue with frija per se. I also downloaded it from samfw and the firmware is the same.

I found a few more interesting things:

1. The firmware G998USQS4AUI1_CL22265714_QB43015853_REV00_user_low_ship_MULTI_CERT_meta_OS11

is what is getting downloaded as the TMB firmware after the September update (includes USERDATA). Installing this results in a Service Provider software version of XAA (SM-G998U_OYN_XAA_RR_0003) and no TMB page during boot up. Inserting a TMB SIM doesn't reboot the phone or give you the TMB specific messages attributes. Installed with unpatched Odin as I mentioned in my previous message.

2. I could not go back to G998U1 firmware without patched ODIN because the regular ODIN quit with a SHA error. So had to get the patched ODIN (details on XDA).

3. Repeated step 1 with the patched ODIN, with the same result. No TMobile page during bootup.

4. Used patched ODIN (the link is on here at XDA) to flash G998U1 unlocked firmware with TMB CSC. This is different than a G998U1 firmware with the XAA CSC.

TMB CSC: G998U1UES4AUH9_CL22265714_QB42384967_REV00_user_low_ship_MULTI_CERT

XAA CSC: G998U1UEU4AUI7_CL22265714_QB43151031_REV00_user_low_ship_MULTI_CERT

5. The TMB CSC firmware has a date of August, 17th 2021. The XAA firmware has a date of September 7th.

6. With the G998U1 firmware with the TMB CSC, without inserting the TMB SIM card, the service provider software version is: G998U1_OYM_XAA_RR_0023. I connected the phone to my computer and checked it with SmartSwitch. SmartSwitch said there was a firmware update available for the phone but, I didn't update the phone. The CSC was XAA/XAA/XAA.

7. Once the TMB SIM is inserted, the phone rebooted as it should. Still no TMB screen during boot up. After bootup, the service provider software version became G998U1_OYM_TMB_RR0016 and the CSC was TMB/TMB/XAA. Connecting to my computer and checking it with SmartSwitch told me that there was no update to this firmware (the reported version was UH or something similar). I reset the phone but the service provider version didn't switch to XAA because when I inserted another TMB SIM card, the phone didn't reboot at all. I didn't have the chance to check if the messages app will have the TMB options. I am not even sure if the phone will get TMB OTA updates. Since this was going to be another family member's phone, I had to give it to them already. Will monitor what happens.

8. Overall not straight forward at all and I don't really know what exactly is happening. I guess I could get access to the boot logs, but, haven't been able to chase after this.

Would really appreciate feedback from experts.

So, my conclusion is as follows:

1. One should only use the very next update to what is already on the phone (for me it was May 2021), so I should have just flashed the June 2021 TMB firmware to begin with. This would have given me a number of firmware versions to upgrade to. I don't think the firmware rolls back even though ODIN might say the flashing is a success. I could not roll back the September version at all even though ODIN completed everything successfully when I tried going back to the earlier version of the TMB firmware from samfw.

2. I am not really sure that firmware available is appropriate or what it is claimed to be 100% of the time or if there is anyway to independently check it. I had a major issue when I tried going back to stock firmware on my unlocked and rooted G955F/DS a couple of years ago. I had at that time downloaded the stock firmware from multiple sites and all made my phone locked to some unknown carrier and I couldn't use my TMB or any other international SIM on. So I had to go back to non-stock firmware after struggling for two weeks. But, with an update a couple of months after that there were no issues going back to stock firmware on the G955F/DS and no lock issues.