Q: “No Service” Beside the Wi-Fi icon

Greetings Rakib World BD. Welcome to Apple Communities! 

I can totally relate to your iPhone 6s not finding a data connection and in my case it boiled down to a carrier issue. Let's try to get to the bottom of this together by following the steps outlined here:

If you can‘t connect to a cellular network or cellular data

Take care!

I want to make sure you've also tried restoring your iPhone. Restoring and reset are two entirely different steps. Also, it can't be done correctly by going through your iPhone settings. It must be done connected to a computer that has iTunes installed on it. Each of the steps provided are very important to keep your personal data safe but to also install a fresh, clean copy of the iOS.

1. Make sure iTunes is updated to the latest version on your computer. If iTunes hasn't been installed  on your Mac or PC, please download and install iTunes from here: iTunes - Everything you need to be entertained. - Apple. 

To see if you have the latest version:

-On a Mac click on iTunes from the menu bar menu and choose "Check for Updates...".

-On a PC click on Help at the top of the iTunes menu bar and choose "Check for Updates...". If an update is found and installed, please restart your PC before continuing.

2. Transfer Purchases - Redownload or transfer your iTunes Store purchases from an iPhone, iPad, or iPod to a computer - Apple Support

3. Import Photos - Import photos and videos from your iPhone, iPad, or iPod touch - Apple Support

4. Back Up - How to back up your devices using iCloud or iTunes - Apple Support -- Since erasing your device through iTunes removes even the operating system, failure to back up your phone will result in data loss.

5. Restore your iOS device - Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

This time through the restore process, set up your iOS device as new and don't restore your back up just yet. If you find you're getting a data connection, go ahead and restore your device from your back up -- Restore your device from an iCloud or iTunes backup - Apple Support

If you're still not receiving a data signal, you should have your iPhone evaluated. You can use the following link to locate the nearest Apple Store or Apple Authorized Service Provider: Apple - Find Locations

Take care!