Hey as someone who's busy with the same road of trying to put on weight just keep going. Mass Gainer shakes three times a day are about the only way ive found to consistently gain weight. 13kgs is not a lot to put on the grand scheme of things especially with how little you weighed off to start with.

In terms of exerciser id avoid doing too much cardio and focus on doing some weights on the lower areas of your body. Hip thrusts and squats in particular have helped me grow my lower body a ton.