jeroent Try to reset network settings in the Android Wi-fi interface. Normally you should be able to connect to your iPhone hotspot without problems.
Tutorial - How to auto enable hotspot from the iPhone
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.
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jeroent 
This is where you reset the network settings (second picture). Try this and let me know the result.
@MihaiFlorin I found the network settings and restored them, but I'm still experiencing the same issue. I'm giving up; I think I'll be installing a modem in the car soon. Thank you for your help; much appreciated. 
surfer63 thanks for your input; I have no idea how to find out how many connections are allowed. I've shared internet via hotspot with multiple devices before.
Is it correct that the DHCP Pool is cleared when performing a network reset on the iPhone: Go to Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings
This is what I tried before and didn't solve the issue sadly. : (
jeroent
I reacted from Android perspective. It seems that iphones limit the number of hotspot connections on bandwidth and performance of the iphone. Somewhere between 7-10 simultaneous connections.
There were issues with carplay on dudu7, which were solved in the latest 3.6 beta. I have no idea whether there is any connection with hotspot for iphone.
19 days later
I have exactly the same issue, with latest update (non-Beta) i cant connect to my iPhone XS hotspot, while regular Wifi is working. Also any other device can connect to my iPhone with no issues. https://youtu.be/7OpsvsEFc0U?si=rPRsXVGoleHRdNt9
https://drive.google.com/file/d/1P3zPLlPBzIHGWCLP1pv6cjMEuPWIQIjm/view?usp=drive_link
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boogiboy Do you have beta iOS? Or do you use Carplay? Carplay and hotspot exclude each other. If you use one, you cannot use the other. Sincerely, since I did this tutorial I never had such problems. It connects in 99 cases out of 100. But I do not use Carplay. There is a 5-6 seconds delay but it connects. Another thing that I can think of is to check if you have Carplay enabled in the BT settings and to disable it.
Just to add to this, this is only applicable to wireless CarPlay.
You can still use Carplay and hotspot together, but it must be wired CarPlay, it’s my current setup for when I use CarPlay once in a blue moon.
MihaiFlorin yes, i’ve tried disconnect CarPlay and even disable that in bluetooth, its still having IP configuration failure, looks like its stuck somewhere in the software on the new update.
boogiboy I have the new update. And I don't have these problems. Try first to check if the hotspot is activated on the Android settings. And second thing I would try a factory reset of the DUDU device.