I don’t use a “tethered” app with my S999+ with Android 11. I install things either through the Play Store in the device or an .apk transferred to the device. I treat it like a fully stand-alone device.
Maybe if you reach out to Rainbuvvy they can provide their S999 image to flash to your device, or whatever image they are using on the S999’s they are selling.
Generally speaking, it’s extremely difficult to get an iOS app to work with Android.
If it has been withdrawn from the Apple store, it indicates that it doesn’t meet security standards. Watch Droid (Android) is appropriate, but of course, on Android devices only.
I found that it is working with application “BT Watch” but the application is too small(function only to resend notifications) . Also I saw that someone has origin rom “4g_h19_s999_en_v0.9_p_user_20230712” and I think that this rom is working more better with IOS (cause I have much older version). So please share it.
Maybe check out here? Seems to be quite useful for this device….