How to help puppies gain weight?

Not sure if this is the right place to ask this but i have 2 8 week old puppies i got at 6 weeks they were strays and extremely underweight. I took them to a vet and asked about their weight but they were more concerned about some respiratory issues they were having so didn’t address the weight at all. They’re shepherd and catahoula mixes so they’re medium/large breeds but i can see all their ribs, their hip bones stick out, their spines are visible, and their bellies almost cave in when looking at them from the top. I feed them the recommended amount for their age but i just don’t see much of a difference in weight. How can i help them gain weight faster? How long will it take for them to get back to a healthy weight?