Default Safest way back to stock

Hi guys, I have european e975 and would like to go safely back to stock, but i need help.

I tried to flash the new V20A stock KDZ via vietnamese easy method, but the tool just can't connect to the device, i tried everything, even fidling with COM ports.

Also I made backup with TWRP of V10H, but now the phone has CWM and CM11 and i don't know if i should use the backup.

tl;dr: Safest way/method to flash back to stock?

ThankYou )