What, do you expect a big flashing neon arrow and a big band playing beside it? Keep looking.
Of course I don't expect a flashing neon "push here"

If I opened this thread it was wondering if the mission was buged because of the alpha, I was not looking for an easy answer.
Finally and about 3 hours searching I found the lever! It was in front of me all this time
Thank both of you for your help!