Unable to activate iPhone 7 in an older IOS

While you have no service you will never activate it. It needs a carrier update. If my advice of hot SIM insertion did not work and your cellular carrier cannot help, and use of another SIM does not help, then you can reach out to Apple Support directly. Depending on your region, you may be able to reach them here: contact Apple Support 

Yes, I visited Apple store and they stated that the carrier module is not working. And then, they asked me to pay $319 + tax for the repair service since it is out of warranty. I found the rod ridiculous. My phone had been working fine prior upgrading to the latest IOS.

In my opinion, this is the problem with the hardware. They had the recall in the past and now they refuse to repair it for free.

https://support.apple.com/iphone-7-no-service

With the power on, take out the SIM and put it back to stimulate the carrier update required.

If it shows No Service contact the carrier call center .

They only repair iphone 7's with the model numbers and dates with the known faulty parts. Yours just died of natural causes.

That’s the model of my phone. And the phone stopped working after the IOS update, not by the natural cause.

That happens sometimes.

Light bulbs usually blow when you turn them on. It's not the light switch that does it, it's the additional stress that is the last straw for an already weak component.