[Q] HELP PLEASE I can't enter download mode or fastboot but can enter twrp and boot! topic

HI! I recently upgraded to an lg g3 and I immediately installed twrp, and put a custom ROM on, now I want to switch back to the stock firmware. I don't have a backup of the LAF file and I can't enter download mode and fastboot isn't working. When I try to enter download mode (holding down top volume key and plugging into PC [I've tried both usb 3.0 and 2.0 with original cable]), lg logo briefly flashes on screen and blue led flickers then the screen is black and the phone shows up on my computer in device manager as Qualcomm HS-USB QDLoader 9008. I wiped my phone through twrp and restored my early backup from when I had just rooted it but that didn't change anything. Now the thing is, I have twrp installed and I can enter twrp just and the phone boots into any custom rom just fine.. The phone is rooted and has twrp recovery. I need to revert to stock but I can't without download mode or fastboot. Any help would be really appreciated! How can I fix download mode? I'm on an lg vs985. ADB works as well.