This is something I haven't even researched yet, but are 5G modems (the ones that seem to be more M.2 slot style) also using USB, or are they finally running over the PCIe bus?
Some have a PCIe mode, but as of now you are stuck with crappy vendor drivers (if any). Default is USB. Lenovo had an issue with their crappy notebooks in this regard. Whitelist triggered if you used it in USB mode.
Seems like there would be no reason to upgrade to PCIe, since (I assume) it would be a pretty expensive update for no meaningful change in theoretical max speed.