My iPhone is updated & it’s iOS 11.3.1 but in my iPhone why not available 4G ?

You may need to update the carrier settings that are provided by the carrier. Here are the instructions for doing that: Update your carrier settings on your iPhone or iPad - Apple Support

I've seen other reports from Grameenphone users that they were able to make a 4G connection after updating to carrier settings version 32.2.

Apple has nothing to do with your connection speed. 4G is part of your wireless carrier's service. You need to be talking to Grameenphone, not to Apple.

Grameenphone announced LTE service in February of this year. They have barely had time to implement it, much less update all of the towers and send out the carrier updates to your phone.

The iOS version does not matter when you consider connection speed. The carrier has to make the connection speed available first.

It's got nothing to do with Apple. It's down to your carrier and the availability of 4G wherever you are.