A bit late to the party, but I'll add some notes that may help future readers.
I was told that the black SIM provided by US Mobile (called "Warp 5G") would not work with the PPP. Not sure if it does or not--I chose the white SIM provided by US Mobile in the same packet (called "GSM"), and it seems to work. It is possible that the black SIM might also work in the PPP (i.e. I may have been told incorrectly information).
For certain: Warp 5G (the black SIM) is for the Verizon network; GSM (the white SIM) is for the T-Mobile network.
I am not using the "pwg" APN that US Mobile calls their "Legacy GSM Network" on this instruction page:
https://www.usmobile.com/help/articles/243520
I am using the latest "wholesale" APN just fine. If I did not put any APN into the PPP (running Mobian), no cellular data would pass at all, though I could make and receive calls.
Re: the whole "stand alone modem' thing.
That's because the Quectel EG25-G modem in the PPP is registered in the IMEI database as being "just a Quectel EG25-G modem" and not under a specific make and model of smartphone or tablet. You could buy a, say, Samsung phone that has the exact same Quectel EG25-G modem in it, but its IMEI would be registered as, say, a Samsung Galaxy B21 in the IMEI database. The carriers COULD look at that and say, "hey, my supervisor told me that we don't support IMEIs that don't say Samsung Galaxy B21," but from a technical perspective, it is the same modem inside.
Put another way, if Pine64 paid to have its PPP IMEIs registered as "Pine64 PinePhone Pro [blah] Edition Smartphone" so that the tech support people see that when they do the lookup, then this objection would no longer be thrown our way by carrier tech support (though they might say, "we don't support PinePhone smartphones" as a way to blow us off).