So I managed to make the ground plane work on my OnePlus 6 the other day.
Literally, there are like 3 steps to it.
I switched my scene up a bit, building with an image target instead. Then I switched my scene back to ground plane...but the issue is...
When I build it again, it doesn't detect any ground plane. The OnePlus 6 is apparently an "unsupported device" for ground plane...even though I got it working the other day!