@ImtiazAli Thank you for the reply.

However the link to first page doesn't apply, because the bricked deco is seen as offline. So updating through the WEBui can't be done.

The 2nd link does apply, but I run into the problem where I don't exactly know which IP needs to be filled. The info and the faq aren't clear about that. I tried

Set a static IP address of 192.168.0.2/255.255.255.0 on the PC’s ethernet adapter  but without success. What I do is:

1) Set the static IP-address (see above)

2) Unplug the deco from power

3) Plug a UTP-cable from the deco to the pc

4) Press the reset button

5) Put the power back on while holding the reset button
Yellow led light --> after 7 seconds ,light turns off while pressing the reset button and doesn't turn on again (or do I have to wait longer?)

I have no clue what to do next. Hope you can help.

  @David-TP Thank you for the answer. After trying your suggestions the problem wasn't solved. I contacted the seller and they are sending me a new M4 for free.

Good to know this forum works great if there is a problem.