This happened to me two days ago. I installed a wrong firmware and my phone lost IMEI number and baseband. You should search for csc code. You can find it in download mode. Then go to sammobile or any other website and look for correct firmware that matches your build number. Search using your csc code ( region of galaxy s8 model). Once you download the firmware. Install it using odin. And use non home csc file. It will wipe your data. But you'll get your IMEI number back.