We have another report of a GS290 which will not start following Easy Installer reporting success. (Reported by @tyru1 in another post as “tout neuf”, brand new.)
I see no documentation for recovering the phone in this situation.
In Gitlab #3429 GS290 bricked @Anonyme linked https://community.e.foundation/t/echec-installation-gigaset-gs290/30792/7 but this now returns “Oops! That page doesn’t exist or is private.”
This page Installing /e/OS after removing UbuntuTouch on the GS290 was reported in this post in the thread Stuck on fastboot flashing unlock(Now Black screen) as successful, we do not know if @tyru1 is in exact same situation.