DIAGNOSIS The 6GHz band (Wi-Fi 6E) likely got disabled, either by a setting change, a firmware update glitch, or because it detected a conflicting radar signal (this is required by law).
FIX Log into the router's web interface. Go to Wi-Fi Settings > Wi-Fi Bands. Look for the "6 GHz" band section. Ensure the "Enable" checkbox is ticked and that the network name (SSID) is set. If it was enabled but the name field is blank, re-enter it and save. Also check if "Smart Connect" is on—if it is, it might be hiding the separate 6GHz network name in favor of a single unified SSID.
IF THAT DIDN'T WORK A radar detection event (DFS) can force the 6GHz radio off for a period. The fix is just to reboot the router. Unplug it, wait 30 seconds, and plug it back in. It should re-scan and bring the 6GHz band back online if the coast is clear.
HEADS UP Remember, only Wi-Fi 6E-capable devices can see and use the 6GHz network. If you turned on "Hide SSID" for that band, you'll need to manually add the network on your devices using the exact name.