I can't remember, Coldplayfan, but I think you are on Android, right? I am beginning to think most app/connection issues are with Androids only. I don't own an Android, so can't comment. Something to do with the DDPai needing wifi AND cellular connection simultaneously?
My iPhone 6+ works perfectly with my M6+ & App for over 1 month now.
At the end, if you replaced the M6+ with a non-wifi dashcam (say the A119 or SG9665GC), you could just the same keep the M6+ and use it like a non-wifi dashcam and don't bother with the app at all. Problem solved!
The M6+ can function fine without the app or wifi. It only needs it for setup & firmware. Just use/borrow an iOS device for that (iPad works fine too).
Also, if you kept your M6+ & waited, you don't deal with hassle and cost of return. Maybe the Android issues will be fixed in future updates. DDPai seems to be pretty active with updates so far.
Just my thoughts...