Just a silly remark, when flashing TWRP did you name the version you are flashing, e.g. "fastboot flash recovery twrp-3.4.0-2grus.img", next command is also with the full TWRP filename "fastboot boot twrp-3.4.0-2grus.img", then "fastboot reboot". The TWRP file should be in adb folder.
I had...