Download twrp from here
Download LineageOS 17 from here
Boot phone to boot menu then fastboot twrp
sudo fastboot boot Downloads/twrp-3.3.0-0-sailfish.img
Wipe to factory defaults
Side load LineageOS Android 10 from the Advanced page on TWRP
adb sideload Downloads/lineage-17.0-withMTG-20191217-UNOFFICIAL-sailfish.zip
Reboot and setup.