How close is the device to the modem?
What type of device?
If you move the device closer to the modem, are you still not able to reestablish a connection?
If you have a LAN connected device to utilize, are all of the check marks green on the System Status page of the System Control Center?
What type of computer do you have? I was having the same issue with an older mac running El Capitan, I switched to 5ghz band and haven't had a problem since.