If you didn’t make the change to FPOpen yourself then you’re under FPOS.
Did you install TWRP yourself?
You have to:
- Download the Android SDK Platform Tools
- Extract the downloaded ZIP file to somewhere you can find it
- Open a Windows command prompt (cmd) or a Linux/MacOS shell and change into the platform-tools directory, which got extracted from the archive
(instructions taken from here)
Now when in TWRP try adb devices, you should get a device.
Then try again the instructions given above.
Otherwise you might find the zip file in the cache of your phone, then you can just flash it from TWRP (in TWRP go in Install, find the cache directory - I don’t remember where it is anymore - and see if the update is here).