@Kevin_Z answering your questions:

1. When the router reboots, are all the LEDs turn off then on automatically and the internet on it will temporarily down then recover? How long does this process take?

Yes the are turn off and the internet is down It takes 1-2 minutes to recover

2. Your network layout, how is the C6 connected to your network? Are there any other range extenders or switch in the network? If yes, try to remove it first and confirm again.

This router works as WDS on 5GHz wifi network. It is connected to another C6 router

3. Who is your ISP and what kind of modem are you using? Is it a Dynamic IP setup on the C6 WAN or PPPoE?

As mentioned above - this router works as WDS. It is connected to another C6 which uses dynamic IP setup

4. How many devices are connected to C6, wired, and wirelessly? What are they? Is there any new device connected to the C6 network recently when the C6 begins to reboot? Or did you upgrade any system in the network before this issue happen?

There is only one deviced connected to it using the ethernet cabel

5. Is there any other setting or feature in your network that may cause this restart issue, or is there any special operation/activity in your network that causes the C6 to restart, such as high loading or real-time calls?

It happens randomly - sometimes multiple times a day, sometimes multiple times per hour. Only specific setting in my network is this WDS

6. Is it the original power adapter on the C6? Is the power stable in your house? How about other electrical devices in your house, are they stable?

Original adapter, connected in the same way as PC that is plugged through ethernet cable

7. Does this issue happen when you purchased it or happens recently? Try to reset the router to factory defaults and reconfigure it from scratch, then confirm again.

I purchased it recently, for a few days it worked fine. After 2 weeks problems started to occur. Resetting router to factory defaults does not work.