Hallo @Emmanuella Nodari 

You can try if setting the Box 12 to WiFi 5 for 2.4GHz will help.

Go to WiFi Settings, change the Wireless Mode to 802.11b+g+n (2.4GHz), this will also have effect on the Extra WiFi network.

@GeSp Thank you for your reply!

However I already have the settings on that mode and still it does not work. 

Do you have any other advice?

@Emmanuella Nodari 

Yes, that's the default (WiFi 6), you should change that to 802.11b+g+n (WiFi5) some equipment doesn't work with WiFi 6

@GeSp Do I need to change both to 802.11b+g+n? Cause right now I changed only one of them and it stil does not work :(

If the SmartPlug only uses 2.4GHz you don’t have to change the 5GHz setting.

You can try it but if don't think it will make any difference, WiFi 5 for 5GHz is  802.11a+n+ac

Did you also try to connect it to the Extra WiFi network after changing the 2.4GHz setting?
 

@GeSp Thank you so much for all of your advices.

Yes, I tried to connect the extra wifi network after changing the settings, but still it does not work, it says on the Wiz app that I have a problem with a “transmission of UDP on my network”, do you know what I could do?

@Emmanuella Nodari

During setup of the Plug your phone has to be connected to the same (extra WiFi) network, I presume you are doing that?

Or else you could try this, found this on there website, how this actually is connecting a Plug to the network is unclear to me:

@GeSp Thank you so much! It worked!