Why does my Galaxy A5 report "mobile network not available"?

[AC Question]

Hello. I have had no issues with my Galaxy A5 mobile until few weeks ago. All of a sudden it began to show the "No signal" indicator on the top bar. I have been looking for tips on how to resolve the problem all over the web and my attempts have been unsuccessful. Checked whether the problem is with my sim card. My phone reported the same when I insert other sim cards. My sim card worked when I inserted it in another phone. I can look at the contacts on each of the sim cards and the phone doesn't seem to have problem reading the sim card.
I highly appreciate your suggestions!!!