Once you've accessed Fastboot mode, you can verify that you're in Fastboot mode by:
fastboot devices
If your hardware buttons are acting up or you're already connected to a PC for development, this is the most reliable method. Enable Developer Options Settings > About Phone Build Number seven times. Enable USB Debugging Settings > Additional Settings > Developer Options , toggle on USB Debugging Connect to PC : Plug your into your computer using a high-quality USB cable. Run the Command how+to+open+fastboot+mode+on+oppo+a74+best