Ok so that rules our WiFi, so There is a problem with the router or between the router and ISP modem...
Next step would be to factory reset, reload FW, factory reset and setup from scratch if you have not done this already...
I'd also recommend that you have your ISP check the cabling going to the ISP modem, check signal levels going to the ISP modem. Check for t.v. line splitters and remove them as they can introduce noise on the line and lower the signal going to the ISP modem. Connecting to the ISP modem could result in a false positive as the signal to the modem could be just enough to that point then adding on a router, could see problems. The router operation is dependent upon getting good data flow from the ISP modem and the modem is dependent upon getting good signal from the ISP Service.
An extreme test would be to take this router to a friends or families place and test it there, if the problem follows, then you'll know the router is bad. Get in contact with D-Link for an RMA. If it doesn't follow, then the problem is with the ISP services or ISP modem.