I tried to reinstall the firmware but in the recover mode I wasn’t able to connect to “192.168.1.1″. The problem is that in this state you cannot reflash your router. Then I googled and googled and googled. After a while I found three possibilities to solve my problem.
The first one was to clear my NVRAM. Because the author of that code used another distribution I couldn’t execute it 
Even if it worked for him it wasn’t guaranteed that it would have worked for me too. Furthermore I’m still a newbie in terms of Linux especially OpenWrt. In fact I should have been able to connect to “192.168.1.1″ after cleaning my NVRAM but what if not? Then the router would neither run correctly nor it could be reflashed. To not brick my router I looked for a better solution.
The second solution wasn’t really better. It was about deleting the NVRAM by a “hardware reset” with a special sequence of pushing and releasing the reset button in combination with unplugging and plugging the power supply. Of course Murphy’s law was true once again so it didn’t work 
So far I didn’t know that I can delete my NVRAM and install a new firmware in only one step. The last solution I found disabused me. The code looks like this:
cd /tmp
wget http://www.example.org/original.trx
mtd -e linux -r write original.trx linux
Congratulations! The new firmware has been installed 
Update: If the router is bricked you can try to open it and short some pins in order to reanimate your friend
