Speed testing is only recommended on wired LAN connections as wireless has many variables and impact accurate results.
What your seeing is good speeds if your on the wireless.
Are you using Single mode N on 2.4Ghz.
Preferred security is WPA2 and AES only. Some adapters don't support AES so you might have to use WPA or WPA2 and AUTO.
For 5Ghz to work and be seen by the client, the client device needs to support 5Ghz radios. You need to refer to the Mfr of the device and check to see what is officially supported. Do other devices you might have support 5Ghz?
Good information on Dlink forum site regarding the DGL-4500. There is a couple of FIOS stickies regarding that and the DGL-4500 on the fourm.
Such a great router.