@Gertjan Well, by unsupported in these contexts, I mean: the phone was not designed for market in the Americas, so the frequency bands on the modem are not the best match (I'm always getting 1-2 bars of reception basically), and Jolla, the company that owns Sailfish OS, considers people in the Americas to be "on their own" as I understand it, despite officially supporting/maintaining the software for the Xperia 10 III specifically. So I have the community support in their forum as my only fallback. Something I see in their forum is that problems with MMS are not considered a serious issue for them because it's being phased out in Europe, or it's insecure, or something like that.
Anyhow, specifically it is only the uploading and downloading of MMS content that doesn't work. I'll still get the bubble telling me I have an MMS to download when one has been sent to me. Cellular internet, SMS, and voice calling all function normally. - And at least nowadays, MMS content gets counted against my monthly cellular data usage, and will not send or download with cellular data disabled.
I'll bring up the network stack idea at the other forum. Is a messed-up stack something that could be fixable?