I have exactly the same problem as you describe. Operation failed and then a series of "device not supported" when trying to add it once again. I am using iPhone.Thanks a lot @David L for your kind help. I appreciate it so much. I have tried airplane mode before and only activated WiFi but Given the information you gave me, I've been running a couple of rounds but unfortunately without any progress. I even took out the SIM card to make sure. I have Android version 9 but I have also tried an older Android version on another Android device and I've also tried my friends iPhone.
After that the doorbell camera device has confirmed connection to my WiFi I have checked that the Android phone has access to my WiFi and it doesn't seem to be the problem. I have also tried to reconnect manually.
Earlier today I scratched my head about the 8-24V DC as well
I called customer support (+1 833-236-6561), they asked me to forward ports 80 8000 and 8080 to the Camera IP address and turn off firewall, to no avail.
The camera is working fine it seems, I am able to connect to it via Blue Iris but the application still gives me errors and a headache.