@David-TP 
Thank you for the reply. I was not able to change anything in the app because when I logged in, it started directly with the set up menu. Same as when you first install the Deco.

It seemed all settings were lost and I had to install everything from zero.

I did not delete and re-install the app as to me it felt that this would not help because the app itself worked well.

Perhaps a little more info: I noticed one device was not connected to the internet so I connected the device to the guest network. 

Few minutes later I got a message from the same device that the connection was lost
When I checked the app, it gave a pop up. Not sure what it mentioned but the app was not able to save or reset. 

I reset the deco's manually with the reset button and when I checked the app, all settings were gone.

When re-installing, I noticed that I could not use certain characters for the password anymore.
Has nothing to do with it probably but it could mean my old password did not work anymore?

  @David-TP 

Thanks and I checked the link.

For the reasons:
1) the ID is my email address so this was ok
2) pretty sure I did not delete the network when I logged in to the app. I tried to change the setting in the guest network to 2.4 GHz but this did not work and I closed the app. 
3) I did change the password when I tried to log in to the app as I forgot it but when I logged in, the network was still there (see point 2)

Solution:
1) done

2) done
2.1) this did not work as when I logged in, the app mentioned: "Paint Your Home in Wi-Fi and let's begin"

2.2) see point 2.1

As I set up the network again, it is working now and yes, I can change settings but still strange that just like that the whole network was gone and I had to reset it.
Is there no option to store the settings and to restore the settings from the app when something like this happens?
Something like a back-up?

And for the characters, I do not know of the space but I could not use the characters # and %