can i repaire or change my iphone 7 in istanbul retail store?

You’ll need to ask Apple for the answer to this.

If it’s an Apple Store or Authorized Apple Repair Provider, and there are no other issues, the recall should be free—but this iPhone wasn’t sold in Turkey and the warranty is not international, which may be a problem for this repair.

Details: iPhone 7 Repair Program for "No Service" Issues - Apple Support

That is a US model phone, and Apple is only required to repair it in a US Apple store or authorized service provider. You can ask at an Istanbul store, but they can refuse to service it because the won’t have the right parts for a US model phone.

There’s a possibility that the baseband problem is a firmware issue. Go to Settings/General/About and look at Modem Firmware. Is the entry blank? If it is you need to reinstall it by restoring the phone to factory settings, then restoring a backup. Back up the phone first, of course:

1. How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Restore your iPhone, iPad, or iPod to factory settings - Apple Support - follow the instructions to restore using your computer, do not using Settings/General/Reset (which is not included in those instructions)
3. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If the modem firmware DOES have a value you probably have a hardware problem.

I'll just amend my previous answer, as there's a 4 year limit for the repair under that program, and the iPhone 7 was being sold by Apple until September 10 2019. However, that particular model was only sold directly by Apple, so at best it might be possible to claim a free replacement by September 10, 2023.

Seems you got your answer from Apple, then. You might have better success with the linked repair program in the US, if your iPhone 7 is one of the ones covered by the program. But it’s probably not worth transporting it back for a repair for an iPhone 7.

But it's still a US device that you're trying to replace in Turkey, and that's where the discussion will probably end. And not likely for free. If you brought it to the United States for repair, it would likely be offered a replacement as board level repairs aren't performed by Apple.

If you read the terms of the iPhone 7 repair program it says the following:

Apple may restrict or limit repair to the original country or region of purchase.