@Sunshine Thanks for responding back. Please find my answers below

1. May I know if there is any trigger when the router restarts on its own? Such as when you connect a specific device or run a specific application?
-- No triggers, I just have my usual devices that I'm connecting for years now, no change in devices or applications. 

2. If no specific trigger, how often does it usually happen? Like once or multiple times a day?
--Multiple times a day, initially it started to restart every few minutes but now it restarts as and when it wants, even after 10 seconds. Sometimes it is stable, it doesn't even restart and act like everything is normal.

3. Did you enable Reboot Schedule on the router to let it reboot automatically at a specific time?

--No I made sure I had it turned off in the settings.

4. When the problem happens, what is the LED status on the router? It will work fine again after it reboots once on its own, right?

--It just turns off all the lights, then all the lights will come back up with the internet LED in orange color, then Power LED blinks for 10 secs or so, then other LEDs comes back up one by one.

5. How many client devices are usually connected to the router? what are they? and via wired or wireless connection?

--7 devices. All wireless connections. Just laptops, smartphones and a smart TV. Two of the laptops have VPN enabled since it is a corporate device.

6. When did you purchase this router? and the auto reboot issue started since the purchase or just recently?

--I purchased it nearly 3 years ago (2 years and 10 months, I believe). But one thing to note is, I had an issue with the router I got initially, it kind of did what it is doing now, I took it to the service center, after a few days, they gave me a replacement. So techincally the router I have now should be less than 2 years old. 

  @Sunshine Could you help? Im still struggling with the issue.