When you plug it into the computer does the computer make a ding sound that it's connected? If so, then press power and volume up until the disconnect ding sound which indicates it's rebooting and then immediately press volume down and see if you can get into fastboot. The timing is tricky so I've found using audio cues from computer can help if the phone is messed up.
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LG G Watch
LG G Pad 7.0 LTE