How to make the Wifi of Airport Extreme and Macbook go back to 802.11ac?
You can try simply turning WiFi off and then on again from the WiFi icon in the top bar.
If it still doesn't work, you'll need to check if you've made settings changes on the router - or if some source of interference has been introduced to the environment (such as another WiFi access point, or other electronical equipment). If this is the case, you can sometimes work around it by changing the WiFi channel used on the Airport Extreme from its settings.