Huge problem with my iPhone 12

Hey,

Today, installed iOS 26 to try it out. It was obviously a bit laggy but it was fine. But not as iOS 18.5.

I tried to go back to it, I saved on iCloud before going to iOS 26, so I downloaded iTunes on my PC, downloaded the IPSW of iOS 18.5 and started it. It's hell on earth. It started fine, my phone turned off and show the thing being downloaded, but it never started. Then, it stopped, and I'm now trapped in the Apple Recovery Mode screen. I really need help, I tried to restart it, but it says error 5, or 2003...

It never works fine.

Help me out, please, I really need my iPhone 12.

Can't connect on apple site too because I don't have access to my iPhone = My number.