Arris DG4550 - Measuring Signal Strength

I have downloaded an app for iPhone called “iWiFi”.  I have measured signal strength AT the Arris router/modem, model DG3450.  Best is -75dBm. Worst is -82 dBm.  BUT, I  have  -79 to -85 in my office 20 feet away from the router.  And this is sufficient to get decent speed tests, but I would like faster.  I want to add access points, but FIRST, I want to confirm that I have good wifi signals from the Arris router.  I think I should get a reading in the -30’s at least, when I measure AT the router.  I think I should be getting a much higher reading.  My ISP is Hargray, and they supplied me with the Arris modem, model DG3450.  I have confirmed that the modem is NOT in bridge mode.  Any suggestions?  Thanks!   Rick.