Hey Gioni,
thanks for your reply! TWRP is not ready for the GS290. So one need to use the eRecovery (Fork from Lineage Recovery). The version from Dec. 2020 is broken, so no adb sideload possible. Meanwhile I found a newer eRecovery version here.
I didn’t had time to test it…
According to this thread it should fix the issue.
If sideloading will work I would use Magisk zip file to root the phone…
Thanks anyway!
kind regards
Christian