I have an ATT LG V40 on android 8.1. The phone is already rooted and has magisk installed. I have been unable to get TWRP installed so I can flash a custom rom. It seems that being on android 8.1 is causing issues, but I am not really sure. I followed this guide: https://forum.xda-developers.com/t/twrp-lg-v40-judypn.3970111/ and when I send the comand to flash TWRP in the command window is says the flash is finished and my phone boots up, but it doesn't seem to do anything. Any help would be greatly appreciated, thanks.