Hello thomasfromrotterdam,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with the Sidecar feature. We're happy to provide some information to assist.
If connecting wirelessly "Make sure your Mac and your iPad have Wi-Fi and Bluetooth turned on. They need to be within Bluetooth range of one another (about 33 feet or 10 meters)." You can find helpful details in the guide here: Use iPad as a second display for your Mac
Additionally, you started from a great resource. Use an iPad as a second display for a Mac
The last section mentions checking for interference with the help of the article here: Resolve Wi-Fi and Bluetooth issues caused by wireless interference
If the issue continues, test a normal restart of both devices.
Reach out to Apple Support directly if the connection issue remains.
Contact Apple Support
Cheers!