ASUS RT-AC88U having issues connecting on 2.4GHz

Hello every1 , I was googling and stumbled this great forum. This is a great forums just like the xda-developers's forums. That said, I'm having a minor issues. I'm NOT sure whether this is normal or NOT. I just got this router, ASUS RT-AC88U.

Two questions:

1. Issues that I'm having with 2.4GHz band. Once when the devices connected to 2.4GHz then suddenly will automatically switches to 5GHz. My question is, is this normal or is a defected router? I've gone factory reset this...

ASUS RT-AC88U having issues connecting on 2.4GHz