Redmi 6a Baseband unknown can be either Hardware or Software.
Mostly, the issue is of Hardware.
But in case of software, you had to flash with its latest version File to restore baseband. In a case, if it fails then it can be resolved by Hardware.
Demo pic of Reballing & making Jumper:
1- Reball MT6177MV.
2- Make a Jumper under IC to the Greenish layer as in the pic.
Afterwards, device will regain its Baseband.