Ok. I had this same issue. Re-booting a million times did absolutely nothing. The phone was getting soo hot too. I went to the cellular store and they could not help either. I finally managed to get it to work by connecting the phone to my PC using itunes. Basically you connect your troubled iphone to the PC with itunes running.
Then you press volume up, then volume down, then press and hold the power button. Now when the phone reboots, it recognizes that it is connected to itunes and you can then begin a update and restore via itunes.
Just check the apple forums of how to reset an Iphone from Itunes and it should work for you.
(P.S. it takes a long time for itunes to update the iphone with the correct IOS 18 software. Think mine was nearly an hour or so....so be patient)