I don’t have a secured deposit in my credit card. You are right, it won’t increase your credit limit but if you pay more for the amount you need it will be added to your available balance .
You pay your credit card in advance in the amount you need , it will come out as an available balance. You can use it anywhere you want. That’s what I always do with a $500 credit limit only. And alao double check it with your bank for sure.