IMEI and Baseband version are very important because they make possible for a phone to make phone calls, so if you lose them your phone loses its basic feature, calling.
And you are here because your phone is showing IMEI and Baseband version as unknown and you can’t make phone calls as well.
But how to fix it? The only way to fix it is to restore the EFS folder that you must have backed up. The EFS folder contains all the important data like IMEI. If you haven’t backed up it, watch How to backup EFS folder. If you have backed up the EFS folder, just restore it. That’s it.