Asus RT-AC1300UHP- Lan Connects, Wifi shows no internet.

So, my Asus RT-AC1300UHP has been behaving weirdly lately. This has been happening for the last two days. Suddenly, my connected mobile phones will show "No internet", however, a few apps like facebook and Messenger seem to connect. When I go to my router's page, it shows "disconnected" on the status. Also, LAN connection to my PC works perfectly.
So far I have tried
1. rebooting the Modem
2. Resetting it.
3. Reverting to an older firmware.

Any suggestions? It's brand new, barely a month...

Asus RT-AC1300UHP- Lan Connects, Wifi shows no internet.