Try other networks. If you can't find one single 5ghz network that it works with, then reset your wifi settings in system settings -> system -> reset options -> reset wifi, mobile & BT. If that doesn't help then factory reset. If it's still not working after this, then you have a hardware issue and need to contact ASUS for a repair
I have the same problem
My Asus ROG 2 download speed is capped at 2Mbps when using 5ghz wifi.
Still no solution?