Usually the baseband firmware sits alongside Android in a secure partition. Like most software drivers, it contains the instructions necessary for the operating system to be able to communicate with the hardware and get it to perform its function. Without the baseband driver, Android doesn’t know how to communicate with the radio, rendering your phone useless.
Alternatively, there can be corruption between Android and the radio which causes it to not recognize your phone’s IMEI number. This results in no connection. Both causes are usually caused by corruption within Android, usually the EFS folder.
Fixing Unknown Baseband Version errors
Fixing Unknown Baseband Version errors is a little complicated but if you follow this guide closely, you should be fine. Before you go making any changes, backup any data you don’t want to lose on your phone. The only reliable way I know of to get everything working again is a full factory image reset. This results in a total wipe of your phone.