Thank you very much, this helped a lot!
some details who may help others: (note: I'm not an expert)
install DT compiler version from the above link, not from online mirrors.
to do so cd into temp directory, curl or wget the above url and chown +x the file as executable
Then install with apt
sudo chmod +x device-tree-compiler_1.4.7-3_arm64.deb
sudo apt install device-tree-compiler_1.4.7-3_arm64.deb
decompile DTB to DTS as mentioned above.
open the decompiled file and lookup for the interface in the aliases section (e.g. i2c7)
You will find the address (e.g. i2c@ff160000)
scroll down now to this address and verify that status is set to okay
status = "okay";
if not change and re-compile to DTB
As I'm not an expert I wonder if it's possible to change or increase the i2c baudrate to 400 kHz ?
I found descriptions to add
As Armbian hasn't this config file I tried to add the same line to /boot/armbianEnv.txt but it doesn't seem to have any effect.