2G/3G not working: Version of Marshmallow B831

Hi. Earlier I had lollipop version of android on my Honor 6 phone. A month back, I upgraded to Marshmallow version B831. Since then, I am not able to connect to mobile data. 2G/3G is not working on my phone. Please let me know what should I do.

Also if I navigate to About Phone ->>Status, I can see that Mobile data state is saying as Disconnected.

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