You're probably not booting to bootloader mode (fastboot mode) that is why no fastboot command is recognized by the device. The prompt
waiting for device means the computer is waiting for a device connected in bootloader mode.
Try to switch to fastboot with the following:
adb reboot bootloader
Now check if fastboot sees your device:
After that, perform all necessary action you want.
Note: You should have all drivers installed as well.
For LG, the command is a bit different.
Start by allowing the bootloader to be written by enabling OEM unlock in the Developer options. Then:
adb reboot download
Unlock the bootloader
fastboot oem unlock
(This will reset your device and you'll get a line of text saying the bootloader was unlocked whenever you boot the device)
Afterwards, flash the recovery with :
fastboot flash recovery xxxxxxxx.img
Alternatively you can press both volume down button and power button to get into fastboot mode.