Hello! Glad to see you here.
To answer your question, yes, I think you need to put on some weight. BMI isn't everything, but it tells me that you're currently at least 7 kg underweight, which is rather concerning. You tell me that you had a "huge belly" at 47 kg, which is still underweight...I don't know your body shape, but that makes me worried that you may have some serious issues with body perception. I highly doubt you will have a huge stomach if you put on a few kilograms of weight.
That said, the basics of weight gain is to eat more than your body uses. This all comes down primarily to increasing your portion sizes, but I would suggest basing your diet around healthy calorie-dense foods like nuts, avocados, and oils, and perhaps also cheese and meat.
[disclaimer: if possible, this is something that you should discuss with your doctor. I am not a doctor, I am an unqualified student expressing my personal opinion.]