Tello was quite responsive. They replied to my request for help on their website (I selected email, not phone call):
"Kindly note that the issue you are experiencing may be caused by incorrect Access Point Name (APN) settings on your device. Nevertheless, please follow the steps below to fully troubleshoot your connection:
-
Turn the phone off, remove the SIM card, clean it, ensure no dust is in the SIM card tray, and reinsert the SIM card.
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Turn off Wi-Fi and turn on mobile data. Wait about 5 minutes, then try using your service again.
*Note that Multimedia Messages (MMS) will not work over Wi-Fi. They require a mobile data connection to be sent or received. Ensure that mobile data is turned on and Wi-Fi is turned off to send and receive MMS messages.
- Make sure your phone’s software is up to date. This is usually found under:
- Settings > General > Software Update or Settings > Software Update
- Install any pending software updates that your device might have.
- Uninstall any apps you recently downloaded around the time the issue began, especially calling apps, launchers, or home screen replacements.
#########I started here 
(Amended for Brax3, iodéOS 6.9 specific menus)##########
- Check your network type settings:
-
Go to Settings > Network & internet > Internet and select the gear icon to get to Preferred Network Type.
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Deselect 5G (recommended) radio button and select LTE.
#####At this point, my phone worked on cell data, using @wardee’s settings.#####
When I re-selected 5G (recommended) radio button, my internet once again broke…curious, I continued after re-selecting LTE.
- Also, make sure that all roaming is disabled, for both voice and data;
- Go to Settings > Network & internet > Internet > T-Mobile gear icon > Roaming–deselect slider.
- Restart your phone and test the services again.
##I skipped step 6, as I had done it (or something similar) previously##
- If the issue continues, perform a network reset (this will erase saved Wi-Fi passwords):
- Go to Settings > System > Advanced or Settings > General Management
- Tap Reset options.
- Tap Reset Wi-Fi, mobile & Bluetooth, or tap on Reset Network Settings.
Note: If prompted, enter the current PIN, password, or pattern. - Tap Reset Settings to confirm.
- Restart your phone, wait 5 minutes, and test the service.
- Ensure your phone has the correct APN settings.
- Go to Settings > Network & internet > Internet T-Mobile gear icon > Access Point Names
- You should see either “fast.t-mobile.com” (for LTE phones) or “Tello” (for 5G phones) as the default APN.
- Tap the three dots in the top-right corner and select “Reset to default.”
- Then, make sure “fast.t-mobile.com” or “Tello” is selected as the active APN.
- Restart your phone and test the service again.
###This broke cell data. Curious, I continued to step 8.###
- If selecting the default APN (fast.t-mobile.com or Tello) doesn’t resolve the issue, try adding a new APN manually:
- Go to Settings > Network & internet > Internet T-Mobile gear icon > Access Point Names
- Tap the three dots in the upper right corner and select “Reset to default”.
- Then tap “Add” or the “+” icon at the top of the screen.
- Enter the following APN settings exactly as shown:
Name: APN
APN: wholesale
MMSC: http://wholesale.mmsmvno.com/mms/wapenc
The rest of the fields, as shown below, should be filled out. If not, please manually enter them as follows:
Proxy: (leave blank)
Port: 8080
Username & Password: (leave blank)
Server: (leave blank)
MMS Proxy: (leave blank)
MMS Port: (leave blank)
MNC: 240 ##NA for Brax3, iodéOS 6.9
Authentication Type: (leave blank)
APN Type: default,supl,mms
Protocol: IPv4/IPv6
MCC: 310 ##NA for Brax3, iodéOS 6.9
- Once done, tap the three-dot menu in the upper right and select “Save”.
- Make sure the new APN is selected.
- Restart your phone and test the service again.
If you’re still having trouble after completing all steps, please reply with the following information…"
######At this point, cell data worked again.########
##I went back to step 4 and re-selected 5G–SUCCESS##