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.
With Premium+, in the Old Phone Cam app. Both phones need internet, and it does not work in a browser, VLC or Home Assistant. Premium and the one time purchase do not include it. The picture is up to 720p at 15 frames a second. While a phone runs as the remote camera it does not stream to browsers or record clips. On iPhone the remote camera runs only while the app is open and in the foreground. On Android it shows a notification you can stop it from. The phones connect directly when they can, and through a relay when they cannot. On a relayed connection the app asks after 30 minutes whether you are still watching, and viewing pauses after one minute with no answer.
Premium+ includes 30 or 60 hours a month. Each viewer counts on its own, so two people watching for one hour use two hours. Direct and relayed connections count the same. Hours reset on the first of each calendar month (UTC), and unused hours do not carry over. The camera screen shows the hours left. When they are used up, remote viewing stops until they reset, and streaming on your own Wi-Fi keeps working.
On the camera phone, open Remote viewing in Settings and choose Use this phone as the camera. A pairing QR code appears. On the other phone choose Watch from this phone and scan it. You do this once. If a phone cannot scan, copy the code on the camera phone and use Enter code by hand. The camera phone needs the subscription. The phone you watch from needs the app and the code, not a subscription, and it keeps up to 5 cameras, each under its own name. One subscription covers up to 5 camera phones, and a camera you stop using drops off after 7 days. Anyone who has the code can watch that camera while it is on, so keep it to yourself. If it gets out, use Reset pairing on the camera phone: the old code stops working.