My MVNO started being recognized incorrectly as being part of another network years ago, and apparently their engineers have been unable to reach anyone within Apple about the matter and get theem to apply what should be a trivial fix in their IMSI mapping table.
Even Google, a nearly $2T company, operates an MVNO ("Google Fi") that does not work properly on the iPhone, because Apple has not added Fi's GID to the correct list.
Jailbroken users can (in theory) add Fi's GID to T-Mobile's "carrier bundle," and things like Wi-Fi calling, 5G connectivity, etc will work instantly.
So right now, someone on Google Fi with an iPhone and someone on Google Fi with an Android phone can be standing next to each other, and the Android phone could have great signal while the iPhone has zero signal because T-Mobile has no coverage at that location.
I don't know if iPhones now support that; if they do, then there's no longer any technical reason iPhones couldn't have first-class support for Fi.