You can mirror iPhone to FireStick directly by AirPlay&UPnP app, then open iptv app
Firesticks can have their own IPTV apps. There's no need to cast. IPTV Smarters is a example.
Smarters is available for iOS so you could probably just AirPlay from your phone.
You can mirror iPhone to FireStick directly by AirPlay&UPnP app, then open iptv app