Hi just saw your post and I have an AT&T V35 too so this should help:
1. Uninstall any prior version of LGUP currently installed.
2.Head to Post #18 at
https://xdaforums.com/android/general/lg-v35-amazon-project-fi-t3819470/page2
3. Download the ' V35_lgup_modded.zip' ; Unzip & install, then ensure the .dll file is also copied to the COMMON folder (C:\Program Files (x86)\LG Electronics\LGUP\model\common)
-If there's no COMMON folder, create one.
4.Shut down the phone completely(phone should also be unplugged), then get into download mode (With phone off,hold down the UP Volume button and connect the phone to your PC while still holding down the UP button) and release the UP button once you see 'Download Mode' on the display.
5.Now, there's no V35 AT&T Kdz file available online so the next best option is to use the Google-Fi kdz you tried earlier; So launch LGUP & select the Kdz file .
6. Next choose the UPGRADE option and flash.
7. Hopefully, that should bring your phone back to life and if not, try again with the REFURBISH option(will wipe user data etc.)
8.Lastly and very important, though you may be tempted to mess around with additional LGUP options such as PARTITION DL etc.; Remember the V35 is still relatively new & has low support so be careful - So far, cross-flashing seems to be fine except for Korean Kdz files but flashing partitions WILL currently cause you to lose either your IMEI/Serial Number or Both.
Cheers