Have you tried to eliminate some options by connecting with an Ethernet cable? If you then have a stable connection, internet is working and you really have a WiFi problem which could be anything like neigbours, electric motors, neon signs, switched power supplies, magnetrons, flipping atoms in the milkyway, whatever.

If the internet connection itself is bad, you need a KPN technician to come over.

Have you tried changing the WiFi channel? Perhaps a neighbor is using the same channel.

What you could try aswell is using the Wifi manager on the KPN webpage, this utility is going to help to see whats wrong and is going to make suggestions and fixes for the issues at your location. And if the speed is lower then for example 20mbit then you’re getting Free wifi extenders or even an KPN mechanic that can solve the issues aswell. 

*admin: added link

@Ernst-Jan @Dragons  thank you all for your replies!

I went ahead and followed all your suggestions but nothing worked (took meall morning haha). I called up the service center and they couldnt figure out what was wrong either. A technician is on their way now to fix the issue.

Hi @Cdk58, welcome to our forum! 
Let us know how it goes!

@Cdk58 we havent heard from you so I'm assuming the technician fixed the issue!
If not, or you have any other questions you know where to find us!