@rdl85 I was notified by email. Carrier can see what device you use, I guess? I think they only notified affected devices.
My issue turned out to be device specific, not UT related. I use a Oneplus 5T, and the Telus network stopped supporting MMS on many older devices, mine included. Per Public Mobile forums, MMS wouldn't work with Android either on a Oneplus 5T, Iphone 5 and earlier, many other older Android phones.

Freedom Mobile setup:
Freedom has separate APNs for internet and MMS, which my phone seems to need for MMS and internet to function simultaneously.
APNs loaded automatically on UT 20.04, so it all just worked right away. I had to do it manually on my device that still uses UT 16.04. Anyhow, it all works great, with standard lack of proper functionality of Group Messaging.

In case needed:

Internet
APN: internet.freedommobile.ca

MMS
APN: mms.freedommobile.ca
MMSC: http://mms.windmobile.ca
Proxy: 74.115.197.70
Proxy Port: 8080

Note that Freedom uses other networks when outside their own coverage, with free roaming. UT has "Data Roaming" turned off by default. This must be switched on under Settings->Mobile for data to work outside the Freedom network.

@Bunchy are you still around the forum? Did you have success downgrading?
I've just bought an N10 that turned out to be from T-Mobile (didn't expect this, as it's not a carrier where I live), and am debating whether I need to return it.

The readme that @rubencarneiro mentions (and authored - many thanks!) says the T-Mobile firmware 10.5.8 version, BE88CB, is unsupported.
I presume the whole device is an issue, and that it's probably a bad idea to try to downgrade my T-Mobile phone to the Global BE88AA 10.5.7 firmware?

It may still be possible for me to return this phone and hope to find a Global version.

@jam1388 Have you continued using this phone? Any further report on functionality?
Your issue with the phone not automatically switching between 4G and 2G is indeed not normal.. both my past UT phones have done that fine. Maybe it's somehow a result of incorrect firmware?

I'm asking for any reports you deem relevant especially because I just bought an OP N10 online that turned out to be an unlocked T-Mobile version (they aren't even a carrier here in Canada). Trying to decide if it's worth trying on 10.5.8, but the need to manually switch between 4G mode and back would already a major drawback for me.

I may be able to return this phone for a short window, so any feedback on functionality (aside from UT-wide issues) is welcome.