You are probably on a FW version 380.3xxx and you cant downgrade if your not doing this, you need to enter in RESCUE MODE
Follow these steps its for Ac66, but work for all models, you need to do all the steps from the link
Power off the router: Hold RESET button and power on router,keep holding reset til led begin flashing slowy, release RESET button and you are now in rescue mode
https://www.asus.com/support/faq/1000814/
After a lot of trying today it worked! I installed the 59_final firmware and it seems the signal is a lot stronger than before.
There is one thing that I can't find out. My sons laptop can only log in on the 5 GHZ signal. When he tries the 2.4 GHZ signal, he can't make a connection. Do I have to change something in the settings?
I am glad it works finaly, and thank you all for the great help.
Greets Tamaar