Iphone wont re-establish wifi connection & iphone mostly stays in 2.4ghz instead of 5.0ghz

Hiding the Wi-Fi SSID is not typical.  Some devices have documented problems connecting to a Wi-Fi network that does not broadcast the SSID.  Since the Orbi Wi-Fi requires a password to connect, most people see "no problem" broadcasting the SSID.  I recommend leaving it alone.

Individual devices connect to whichever Orbi channel "seems best" at the time.  5G signal does not carry as far as 2.4G. There is a good chance that the iPhone detected the 2.4G band first and connected before the 5G band became strong enough.  Over time, the Orbi and iPhone will realize that 5G offers a better connection, and the iPhone will switch bands without you doing anything.  That is what all of the mesh Wi-Fi systems with a single SSID for both 2.4G and 5G attempt to make seamless to the user.