It’s not the same way to install e-2.7-u-20250112-UNOFFICIAL-dreamlte 
signed dev-keys by @ronnz98 as my first installation two days ago, but now I know it’s the shortest way, and - that there are several ways to successfully install it.
The pivotal point is the recovery. Because the e-Recovery does not work to its full extent, I installed it first:
recovery.img from lineage-21.0-20240619-UNOFFICIAL-dreamlte.zip by @Ivan_Meler
then
lineage-21.0-20240619-UNOFFICIAL-dreamlte.zip by @Ivan_Meler (probably not absolutely necessary)
At first glance, this configuration works as it should.
To install e-2.7-u-20250112-UNOFFICIAL-dreamlte signed dev-keys by @ronnz98 I did the following steps:
A) Transferring e-2.7-u-20250112-UNOFFICIAL-dreamlte.zip by @rozz98 to a microSD card
B) Reboot into the LOS21.0 recovery
C) Factory reset > Format data + cache partition
D) Apply update > Choose from sdcard (microSD card)
D) Reboot system now
E) Wait patiently until the system has initialised.
Rejoice!