remember, your tank has only been up a little over a month. The bacteria that consume nitrate take a while to get fully established as they grow in the deep non oxygenated parts of your aquarium. a DSB
can add a place for anaerobic bacteria to grow but it will still take some time for it to colonize. as others have mentioned, growing macro in a sump can help lower Nitrate as well.
you can vacuum the very top of your sand bed, but dont stir it up, all that does is oxygenates your anaerobic bacteria and they dont like it and it also stirs up debris and releases nitrates back into the water column (if you are doing this with every water change most likely that is why your nitrate will not go down)
Remember, very few things come quickly in this hobby.
one last question that just popped into my head.... you aren't using bio-balls are you? they are far too good at aerobic respiration and leave you with all your Nitrate being produced away from places that will have anaerobic bacteria, leading to high levels of nitrate.