Just bought a DAP-1720 Dlink to get better coverage in

That button with no light is the reset button beyond doubt. Press it, hold it in for 20 or 30 seconds continuously, and you will reset the box back to factory-issue settings.

Now, you will need to know some things about range extenders, first off:

Remember that, in configuring a range extender, one has to set it to the same SSID as the WiFi router one is trying to extend.
And also, the encryption code must be the exact same type (not just WPA, for instance, but if the main router is WPA-PSK, then the extender must be WPA-PSK too. This requirement may even extend to revision level, so if a router is very new and an extender is much older, it may not work.

Another point: range extenders must actually be within range of the main router. So check on that also, by trying to access the main router (after turning Off the range extender altogether, and unplugging it) from that same position with a wireless-equipped laptop, and see if it has a reasonably strong signal, of at least two bars I would recommend (though only one green bar *might* work reliably.)

Now, your DLink is undoubtedly within range of the modem. What means have you used to set it up to the main modem/router's parameters, to match them?

Let me know how it goes, and what you find out, please. Thanks.