Internet issue - Sierra 598u with Asuswrt Merlin(RT-AC66U_378.56_0)

Hello Team,

I have installed "RT-AC66U_378.56_0" on Asus AC66u router.

I'm using Sierra 598u to connect my internet.

It is Connect internet without any problem. but, after few minutes it disconnect the internet. The Icon (on Asuswrt Merlin ) shows connected but no internet. I have flashed NVRAM and Factory RESET as well.

Can someone please help on this issue?

Thanks in advance.