Perhaps the USB WiFi adapter only supplies 2.4Ghz. and the KPN modem only supplies 5Ghz.?

Did you bring that USB WiFi adapter with you to The Netherlands? In that case it might not support 5Ghz. channels that the modem uses. Internationally channels 36-44 work everywhere, for higher channels there are restrictions between countries.

Yes, I brought the USB wifi adapter from Poland. I do now know what it supplies. Should I buy a new adapter? Do you know what kind of wifi adapter should I buy? I have a quite old computer, but it works fine. Unfortunetly is got windows 7 and I cant upgrade it anytime soon. I dont know if it is a problem. Thank you very much for your response.

My tp-link usb adapter is this model I believe - TL-WN821N. I read on the internet now that It doesnt 5ghz as you mentioned. 

I think I will have to get a new one then. If you have something to reccomend I would be glad. Thank you very much :)

Ok. But I believe Poland uses the normal European ITU approved channels. So that can be ruled out.

Do you see some sort of make and type on the USB adapter? Then we can have a look what bands and channels it support. If it is old, it might not support WiFi on 5Ghz.

A really good adapter is the TP-Link T3U.

Yes, the one you have does not support 5Ghz. to which your modem might be configured exclusively. If you have admin access to that modem you could try to enable 2.4Ghz. to make it work.

Or buy the T3U for around EUR 20,-

Highly recommended since it also works on older Windows versions. It is even smaller than the one you have. 

Okay I will buy this one thank you for your support :)