Dan mentioned in a post below that the device going to sleep might be causing a problem with the modem. I suspect it might be poor reception in that particular instance but your issue might be different. In the administration section of your modem there should be an option to stop the WiFi from going to sleep. Make sure it's set to not go to sleep save the changes and see how you go. The picture below is a different modem however that's what you are looking for.
Try http://192.168.8.1 in your browser. Password should be admin or password or it could be the same as your WiFi password. See if that lets you in.
You need to be connected to the modem for the above to work just in case you didn't know.
My login looks like the pic below.