Okay, so, a while ago my dad bought a DIR-615. I just set it up and it works amazing for my brother with wireless. Fast downloads, faster than he used to get too. Me, on the other hand, get 30kbp/s. I'm on cable and I USED to get 500kbp/s! So, what the hell do I do?! D:
First thing is to check the version of the firmware. Make sure if you want to update the firmware that you get the one for the hardware version and update it from the "wired" connection. Also, the DIR-615 firmware 2.25 ot greater has extremely limited QoS support (it is on or off.) Also, you might get more help with the D-Link forums directly.
