iPhone 7 Update/Restore problem: Unknown Error (-1)

I have tried to update my iPhone7 (128 GB) to iOS 11.2.5 but my phone got into restore mode after the update. I restored it with iTunes and it worked. Yesterday, there was a problem with the cellular signal and for half a day I wass unable to use my phone sine it couldn't find the cellular network. After a while, it went back to normal. The same day, I tried the iOS 11.2.6 update but it went into restore mode again. This time I was unable to restore with iTunes. I tried many times, different PC, different cable etc. with no success. It always gives the "Unknown Error (-1)" warning through the restore process when it is at the "Firmware Update" step. Now I am stuck with a phone displaying the iTunes screen and nothing further.😟