Thanks for your replies guys!
Things I tried; but without any success:
On the iPhone
- both tried iPhone hotspot enable / disable maximize compatibility (like @mrqasq mentioned),
- reset/restore network settings (Settings > General > Restore network settings).
On the DUDU
- tried to remove the wifi connection in the list above and reconnect it again,
- tried to use the wifi setting metered / unmetered and automatically,
- tried randomized MAC and device MAC.
Other tests
- connecting DUDU to normal normal wifi works,
- sharing 5G internet from iPhone to other device (Macbook) works.
The error message
@MihaiFlorin I tried to located the reset network settings like on the DUDU; but I'm unable to find these. When I google for these they should be located under (System > Reset ....) But nog in my menu; see; below.