OK, I solved this.
The problem was full DHCP table, because new smartphones do mac address randomization on each connection. Contrary to normal DHCP which assigns the IP address that was not used recently, if Volkswagen DHCP server has seen 255 IP addresses, it just stops assigning new ones.
Quick fix:
Assign a static IP address to your device (if it already happened), or turn off mac address randomization.
The IP address range is 10.173.189.0/24, that means you can assign some IP address, such as 10.173.189.50 (and then 51, 52, .. to other devices). The gateway and DNS is 10.173.189.1.
Another problem is with Android Auto over wifi. This uses a temporary wifi network negotiated I think over USB or bluetooth. This is not configurable on the phone and it will randomize mac addresses and there is no way to turn it off.
If you have this problem, this is a more thorough fix:
1. Reset to factory settings (this clears the DHCP table, I found no other way to do this)
2. If you have GrapheneOS on your phone, go to Settings / Apps / Sandboxed Google Play / Android Auto and disable permission for Wireless Android Auto (so it doesn't connect through that temporary network and take DHCP slots - you can't disable mac address randomization for this temporary network)
3. Keep both keys on you.
4. Run the Volkswagen app on the phone. Delete the car(!) - even though it seems to be possible to pair again, it doesn't work, I've tried many things, you have to delete it, but it's not a problem, it will come back again
5. Create a new user on the car, login via volkswagen ID (only after deleting the car will the login work). It seems that it is possible to add the car through the app - but if you do it, you will not be able to login, so you have to do it from the car.
6. Become primary user - confirm
7. It will want to pair the keys (unlock with one and the other key, as when you first set up the car)
8. Change hotspot settings (network name, password), if you want
9. Connect to the hotspot, remember to disable MAC address randomization in the network settings of the hotspot network.
10. Log in to Cubic Telekom
11. Everything works
Note that I tried many other things and nothing else worked.
