As Gary has said Reboot the Router but have you considered putting the External Devices on 2.4Ghz instead?
5Ghz is good but its range is far more Limited so your extra oomph comes at a trade-off.
2.4Ghz will be a little slower but far more stable when it comes to the distance. You could also try using WiFi Boosters to the other rooms.
I suspect the reboot will fix it but if it was to continue to drop off I'd consider the above
Good Luck