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It was the first and only issue i came across after flashing my Lenovo a536 with new stock ROM. When i finished flashing and rebooted my phone, everything was working quite normal except the SIM cards. The device was detecting both the SIM cards but it was not able to catch the network. Even though i could use SIM 1 network sometimes after restarting the phone again and again, SIM 2 was totally dead. Later when i cross checked the phone totally, i came to know that the problem was Invalid IMEI. When i slide down the notification bar, it was showing Invalid IMEI for both the SIM cards. When i checked the IMEI status of my device using the code *#06#, it was showing again Invalid IMEI.
This is a common issue occurs after installing a custom ROM. If the ROM is with some errors of some issues, it may occur. The solution is simple but the problem is quite big because, without a proper IMEI, your phone is just a device, not a communication device. In the the below article i have explained how to solve Invalid IMEI problem.

How to Solve Invalid IMEI issues after Flashing with New ROM

Thanks for reading. Feel free to comment for any doubts.