Iphone 7 LTE problem , can't switch to LTE mode in LTE mode just showing 3G

Well that would certainly explain why you have no LTE (4G). See the compatibility list below.

https://www.apple.com/iphone/LTE/#iphone-7-iphone-7-plus

What else does it say by Carrier? Something beside the number.

The iPhone 6s is designed for that area, despite the fact that Iran does not currently have a supported carrier.

I would sell the iPhone 7 and purchase one designed for your location.

It is wonderful that you have 3 carriers to choose from. That does not change that these are not Apple supported carriers. What I said was that you did not have a supported carrier. You need to try and figure out what phone your carrier would support. I'm guessing that you are not the only person to want to use an iPhone. The carrier must know something. Just that they do not have a supported carrier file for the device.

What is the full model number when you go to Settings>General>About>Model? And, where are you located?

In what region are you using this phone?

Where are you located?

A search indicates this to be either a Sprint or Verizon phone. Hard to tell with what I've seen.

Well, it appears that the device you have is a North American device, so there is a good possibility that the device will not support the LTE bands in your area.

There really isn't much that you can do. Not a good idea to purchase a phone that is not designed for your area. This is especially difficult when there is no supported network in Iran.

Go to Settings>General>About>Carrier and post what is found there.

Also, what is the Model number for the 6S?