Hi Tim,
so my Fairphone 3 and 5 do work with xDrip+ work quite OK with Dexcom G7 and by the description of the app One+ should work too. So I did not test it specificly (only Dexcom G7).
I am not sure how high your skill level is, but it is at least a little advanced. Here are some steps you can make to try it.
Xdrip, is on your own risk, but opens up a lot more phone to use a cgm,
Here you find the Docu: xDrip+
You need to allow the non playstore sources (as it is an unofficial Open source project).
After installing, You need the specific type of CGM you want to use for Dexcom types:
For me it does work, quite OK, reliability is not perfect, but that seems also the case with dexcoms on app, as far as I hear.
But overall for me, it is the way I can have google free android on fair phone and my diabetic needs fulfilled.
Hope it helps,