Im on insider release ring, quite stable on lumia 640. If w10m releases you can easily change back to production ring from insider app and you will get official build. No need to go back to 8.1
All androids have a "3g only" mode that prevents the phone switching to edge in case of a weak signal. That's what i was talking about. On my phone there's just 3g and 2g. But i have seen screenshots of w10m with that setting.