I'm a nerd with 25+ yrs of working with cursed sand, my mother is someone who asks either of her sons to help with tech difficulties.
After an hour of struggling, I expressed my frustration and the problem to her. She patiently listened to me and suggested taking the rendered video from DaVinci Resolve on Mac that wasn't recognised by YouTube app on Android, run it through my video editor on iPad (LumaFusion) and then transfer it to Android and try again.
It worked! 🤯