One way to fix IMEI Null and Baseband Unknown Lenovo A6000 are to improve the EFS partition by restore QCN file. You can restore QCN file using QPST (Qualcomm Product Support Tool). Your device must be rooted for restore.
Using Port: Qualcomm HS-USB Diagnostics 903A (COM150)
modem build: Mar 10 201512:02:03Feb 10 201512:00:00EAAAANUZ
modem version: DPM.1.0.c7.18-00021-M8916EAAAANUZM-S86518C1-V18
Backing up NV to QCN file...
Using default SPC (000000).
1197 NV items readed.
QCN file saved