The usual snags are below. If your answer is not here, write, a person answers.
Both devices have to be on the same Wi-Fi. A guest network usually isolates devices from each other and will not work. If you typed the address by hand, make sure you typed the key at the end too; scanning the QR code is easier.
iOS closes the camera when an app goes to the background. That is a system rule and there is no way around it. The Dim Screen button in the app takes the screen almost all the way down while keeping the stream running.
That is normal on a long stream. The app watches the temperature and lowers the frame rate itself when it has to, and says so with a badge at the top. Keeping the phone plugged in and in open air helps.
Press Restore Purchases in the settings screen. The purchase has to have been made with the same App Store or Google Play account.
In the phone’s own storage, under Recordings in the app. Nothing is uploaded, so deleting the app deletes the clips with it.
Yes. The stream is standard MJPEG; give either of them the address as it is.