RT-AC86U Speed drops

Hi,
I have RT-AC86U running on Merlin 384.13_0. My Internet speed 1 gigabit from Comcast.I am getting the speed around 400 MB/s when the router works normally, but after a day or to my speed drops below 10 MB/s. Simply reboot puts the speed back to normal.

Please let me know what setting I missed.



Thanks,
Gregor