HOW TO FIX INVALID IMEI

Invalid IMEI

Caution: Always remember that root-related actions are your sole responsibility.

Requirements:
MTK-based android phone in hand
Root access
IMEI number
you can fin it by removing the back cover of your phone and look for IMEI 1 for SIM 1 and IMEI 2 for SIM 2.
it is also written on the box
no luck? use the IMEI from the same phone
Method 1:
Go to Engineering Mode
Dial *#*#3646633#*#*
If the above dial doesn't work, download and install MTK Droid Tools. Open it, select Engineer Mode -- Engineer MTK.
Go to Connection tab
Select CDS Information
Now, select Radio
Select the SIM which you want IMEI to be fixed.
Input the following:
For SIM 1, at the AT+ line, make it look like this (replace the red text with your phone's SIM 1 IMEI number):
AT+EGMR=1,7,"SIM1_IMEI_NUMBER"
For SIM 2:
AT+EGMR=1,10,"SIM2_IMEI_NUMBER"
     7.  Press SEND AT COMMAND to send and save the IMEI.
     8.  The panel will close automatically. If you are done, reboot your phone.

Method 2:
Download and Install Terminal Emulator on your phone
Open Terminal Emulator app
Type SU then press Enter
If asked for Superuser permission, grant it.
Then, type the following command then press Enter.
echo 'AT+EGMR=1,7,"IMEI_NUMBER"' >/dev/pttycmd1
echo 'AT+EGMR=1,10,"IMEI_NUMBER"' >/dev/pttycmd1
Restart your phone.
How to Fix Unknown Baseband and IMEI

If you have done replacing your phone's IMEI using the given method above but still it won't work and also the Baseband was still unknown, follow the steps below..

Caution: Do at your own risk mate.

Things you need:
MTK phone
Root access
Windows computer
MTK Droid Tools
IMEI Number - written at the back of your phone (remove the back cover)
Android USB ADB drivers
NVRAM backup from other device with the same model
Sorry but this is the only safest way. You can comment your phone model so I can help you look for it. Once you had the NVRAM backup, copy it in your computer
Instruction:

(click image to zoom)
Download MTK Droid Tools and extract it on your computer running Windows OS
Download and install USB ADB drivers
Enable USB debugging on your phone (go to Settings -- Developer options) and connect to your computer using USB cable.
From your computer, open MTK Droid Tools application.
The tool should detect and display your phone information.
Click the ROOT button for temporary Root shell access on your phone.

Click on IMEI/NVRAM
Click Restore button, then select/open the NVRAM backup that you got.
Change IMEI, input the ones you have.
Then click Replace IMEI
Restart your phone.
Done
credits to: XDA and to the developers of the apps used.