How do I make paneer firm and chewy like in the restaurant?
If you are using UHT milk, that is your answer right there. It won't make proper ricotta or mozzarella, so I'm not surprised it won't make good paneer either. Something about the heat treatment makes it so that it will only produce grainy, tiny curds. I made this mistake myself once, in my excitement to try my new cheesemaking kit. Here is some reference info, including pictures, from cheesemaking.com.