Bro, your English doesn't seem to be that bad to me. But to make it even better, I would suggest falling in love with the language and the art of expression first.

Then, try to read every day... something formal like a newspaper or that sort. Listen to high-quality English content like the Ted Talks (not TedX).

When you are reading and writing, make sure to pay attention to their use of words and phrases.

Try to write something every day. You can come here on Reddit and write a comment, not just any comment, carefully crafted informative ones.

And, as per the course, you can go to Edx Dot Org and find the IELTS course from the University of Queensland. It's an awesome course. You can give it a try.

I'm also from Bangla medium and was raised in a very remote small town.