Agreed with all above. Public Mobile's "3G" works across a 4G connection.
The main things to worry about would actually be voice calling (since Public Mobile does not carry VoLTE) and SIM compatibility (since Public Mobile doesn't eSIM). Although these, too, are non-issues.
But more to the point: the iPhone 12 apparently will indeed support HSPA, LTE, and 5G. Or at least some regional variants (like the ones which will be sold in North America, USA, and Canada) will each support their specific region-compatible network technologies.