@Jordyvdburg 

Did you factory reset the extender? If you can't see the router's wifi network when setting up the extender, please try the following suggestions: 

1. Confirm the extender is close to the main router, 2-3 meters is recommended 
2. Make sure other wifi clients can see router's wifi signal 
3. Change a different wifi channel on the main router: 
*For 2.4G WiFi,  please change to one of the channels between 1-11
*For 5G WiFi, please change the channel to 36,40,44,48
4. Please reboot the main router and rescan for wireless networks      

Have a nice day.