Need help with reinstalling stock ROM

[branddiogo]

So I'm trying to revive my mother's LG G4 Stylus (H540T), because it has an intermitent message like (com.process.google.gapps has stopped working) that won't go away for a few hours, and then it works ok for another few hours. Also I'm unable to access the camera.
So after factory resetting it and everything else that I found on the internet, I've decided to reinstall the stock ROM, so I installed all LG drivers, downloaded the .kdz file and LG Flash Tool 2014, here comes the issue:

From all tutorials I've read and watched, I need to go into download mode to reinstall the file from my computer, and I've tried to do that two ways:
1 - Hold the Volume Up button for a few seconds, then connect the USB cable and release the button when the LG logo shows up.
2 - Hold the Volume Up button for a few seconds, connect the USB cable and don't release the button until I'm in download mode.

When I try it the first way, I just get into a screen that says "No command".
When I try it the second way, I just stay in a boot loop, where the LG logo comes and goes forever.

Please tell me what I'm doing wrong, what should I do. I don't have any knowledge with this sort of thing on Android, so any help is welcome.