@David-TP I've solved te problem. I found a solution on youtube. I had to give all the devices (phones, tablets, laptop, etc.) a static IP adress. Everything is working fine!

After 3 months the Deco's E4 have a bad internet connection. What I have tried: - reset and a new installation - shuffled the three Deco's (3d floor -> 1st floor -> 2nd floor -> 3d floor) - restart...