@MartienS
Hi, welcome to the community.
"Only from this iPad, the internet connection is always disconnected by deco after a few minutes."
- Would it automatically restore the connection a few seconds later? Or do you need to manually disconnect/reconnect to the WiFi on the iPad?
Please also disable "Beamforming" under Deco APP>More>Advanced; then, turn off "Mesh Technology" to see whether it makes any difference.

After that, please enable the Guest network and test whether the iPad can have a stable connection over Guest WiFi.
Wait for your reply and best regards.