One thing I'll mention though: this is a very old (in "computer years") product. The user manual was published in 2007, and the product defaulted to using WEP encryption, which is seriously flawed and outdated. It also supported only 802.11b or g, not n nor ac. You needed to go into their advanced configuration section to instead select WPA2, not WPA nor WEP.
See pp. 58-62 in their manual. You may also need a firmware update for that modem/router. Better yet, re-configure it to run in bridge mode (making it just a modem, not a router), and use a modern WiFi router instead.