Make sure Roon is working well from your PC, that is: as Control, Core, and Output. I am assuming this is how you have your PC set up; you mention you are using a PC (i.e. not a Mac).
Uninstall the Roon App from your iPhone. Reinstall it. If you have enabled the device (your iPhone) on the main settings page within Roon (on your PC) and the iPhone app still doesn't lock in, then contact Roon.
I don't think there should be any cross platform issues. I believe Roon Remote works fine on my wife's iPhone, while I am PC and Android based.
Uninstall the Roon App from your iPhone. Reinstall it. If you have enabled the device (your iPhone) on the main settings page within Roon (on your PC) and the iPhone app still doesn't lock in, then contact Roon.
I don't think there should be any cross platform issues. I believe Roon Remote works fine on my wife's iPhone, while I am PC and Android based.