Welcome to Android Central! What's the exact brand and model name/number of the device? Are you saying it will only connect to the router's 2.4 GHz band, and not the 5 GHz band? First, make sure the 5 GHz band has a different SSID (name) than the 2.4 GHz band. Second, if you're far from the router, remember that 5 GHz has significantly worse range than 2.4 GHz (due to its higher frequency).