When the app refuses to add a device, confirm the following points one by one.

• Same LAN? During hotspot provisioning or QR-code pairing the phone must join the same router as the device, on the 2.4 GHz band. Most cameras do not support 5 GHz, and joining the wrong band fails immediately.
• Phone permissions: iOS needs Local Network access granted to the app; Android needs camera and location permissions. Denied permissions leave the scan unresponsive or the device undiscoverable.
• Already bound: a device previously added to another account must be unbound there first, otherwise the new account cannot add it.
• Router restrictions: with AP isolation or a guest network, phone and device sit in different broadcast domains and cannot see each other. Disable isolation or use the main network.

Also make sure there is enough light when scanning and that the serial number is typed correctly, then give provisioning a minute or two before refreshing the device list.