LG V30 - Flash ROM using download mode?
For linux use this link it's a tool called SALT. If you're good with Linux there are tools to use with Python such as ESPTools or LGLAF. 
For Windows  look up FWUL, it's "Forget Windows Use Linux", it's a variant of Linux build for this purpose but they were built for the older models.  


The easiest is to use LG Bridge or LG Mobile Support tool if windows or linux recognizes the phone when you plug it in as an LGE Android Net USB Serial device with a com port.  Below is the option for more tech savvy people.  

If you can get your hands on windows try LGUP.  You can google it or go the link  to download LGUP installation files (you can find them on the same forum) .  It consists of 3 MSI the LGUP Store version 1.4, the LGUP Frame and DLL. 


If you need the stock rom for your phone go to lg-firmware.com
Once it's installed, download  this patched dev version from this 
XDA post. 


It has 2 files LGUP.exe and LGUPCommon.dll. Replace the LGUPcommon.dll and overwrite the one on your C:\program files X86\LG Electronics\LGUP\model\common\ (if common folder doesn't exist create it).  Then put the LGUP.exe 2 level up from the Common folder.

Basically, put your phone in download mode first then launch LGUP.exe.  This will give you a few options to flash. 

1.Refurbished will flash then wipe your data meaning pictures and personal files will be gone.  


Upgrade option will upgrade and keep files.If you want to flash different variant/carrier chose the DL Partitions and select all do this with extreme caution because it will brick your phone.  
If you want to cross flash to unlock go on the forum you found LGFLashtool, XDADeveloper, and search for Frankenstein that's the method to root and unlock your phone.


Oh and if you chose the Refurbish or DL Parition you might lose your IMEI and serial number so either back it up first or write it down to do it later.