[Kushan]

Right, nobody's suggested this but I think it's the best, sure-fire way to fix any settings issues that may be the cause: Factory reset the router.

Pinhole reset it so that everything goes back to defaults, then see what speeds you get. IF you're still having issues, contact Asus who will replace the router as it's clearly faulty (Many of us have the same or similar router and have zero issues).

And yes, before anyone gives me grief of "factory reset being the typical virgin response", in this instance it really is the best response. Asking the OP to buy another router is preposterous.