Thanks Eskimo. For now, I have found a workaround: When I use AccessorySetupKit to add the iOT device as an accessory, and afterwards use NEHotspotConfigurationManager to connect to it, the wifi connection doesn't get disconnected after an iPhone reboot.

Yes, the same problem occurs when connecting manually through Settings. After iPhone (iOS 18.3) reboot, I go to settings and connect to the IoT (ESP32) wifi network. The wifi connections gets labeled as "weak security" and "no internet access". Within a minute, the iPhone connects to another wifi network that has internet access automatically. I then connect to the IoT device again and the connection remains intact.