I have an issue with my iphone 6plus where it has been in a constant loop of restarting and I get unknown error 9 when plugged into MacAir for iTunes.

I have an issue with my iphone 6plus where it has been in a constant loop of restarting (screen shows the apple logo for 10 sec then turns off for 10 sec and then does it again - it has been doing this for 3 days straight). I have tried to force restart the phone (holding down side power button + home button) with no luck and I get "unknown error 9" when plugged into MacBook Air for iTunes - when trying to restart or update the device via my MacBook. I have already replaced the internal battery of the phone, so this is not the problem. When the device is not in the constant loop of restarting, the screen shows an arrow pointing from the charger icon to an "iTunes" icon with the phrase "support.apple.com/iphone/restore" at the top of the screen, however, once I try to plug the phone into my laptop the phone starts the original loop process all over again - does anyonw know what is wrong with the phone and how to fix this issue?