DIAGNOSIS Yeah, this one's common. The 2.4GHz radio can temporarily crash, often due to interference or a bug in the firmware's wireless driver.
FIX The quickest fix is a reboot. Unplug the router's power, wait 10 seconds, and plug it back in. Wait a few minutes for it to fully boot. The 2.4GHz network should reappear. If it does, but the problem recurs, log into the router (192.168.50.1) and go to Wireless > General. For the 2.4GHz band, try changing the "Control Channel" from "Auto" to a fixed channel like 1, 6, or 11 to avoid automatic channel-switching hiccups.
IF THAT DIDN'T WORK A firmware issue might be the cause. Go to Administration > Firmware Upgrade and check for an update. If you're already on the latest, you can try a factory reset via the reset button on the back (hold for 5-10 seconds until the power LED blinks). You'll have to set everything up again.
HEADS UP Some IoT devices (like smart plugs) can cause specific interference that crashes the 2.4GHz radio. If the problem started after adding a new device, try powering that device off and see if the network stays stable.