Hi All,
When upgrading from Raspbian OS stretch to buster you could encounter a few surprising things. I’m not a coder what so ever but just a regular user of the Banana Pi M2 zero boards. So my 5 cents input here should not be criticized to much. 
Step 1)
Do your upgrade from stretch to buster (Follow instruction on the web)
But, before you’re doing so switch off your firewall
Step 2)
Do an sudo update autoclean, than sudo update --fix-missing and last sudo update.
Don’t execute an sudo apt upgrade! You will loose all wifi connectivity.
Good luck