I’ve seen a number of NTRIP client apps floating around but I’ve never used them and can’t recommend any. That’s ultimately something I’d like to support in GPSTest too. Let me know if you find one you like.
I haven’t yet seen evidence of decimeter accuracy from any Android phone with integrated dual-frequency support.
FYI, the Xiaomi Mi 9 doesn’t support accumulated delta range for use in raw measurements, so you may want to avoid it or look at other options if possible: