Solved it by using Google's NTP server ip addresses which are:

216.239.35.12

216.239.35.8

216.239.35.4

216.239.35.0

I added a static route to network of NTP server address. Here default gateway address is the ip address of my root/parent router

Then I added NTP server addresses in Time settings

and now auto time sync via NTP server works.

Only issue remains now is IPV6 not working in WDS bridge mode

WDS with WDS1 Mode is working but NTP time sync not automatically working and IPV6 not working.

WDS2 Mode doesn't work in 2.4ghz or 5ghz if configured in child (c6) router only.
If WDS2 is configured in both routers then it makes the root/parent router slow with 1000ms ping from child to parent router.

My root/parent router with DHCP enabled is Archer A6 V3.

Is WDS2 Mode buggy or is there any other setting I can do to make it work ?
because I read online that WDS2 Mode support IPV6 traffic.

WDS1 connection Speed Test:

Parent/root router = Archer A6 V3
Child/Extender router = Archer C6 V2

Distance between root and extender router = 10M
Distance between Extender router and device (Dual band Android phone) = 10M

PC is connected to root router via Ethernet cable
Phone is connected to Extender router via WiFi

I am transferring a file from PC to phone

Speed with WDS1 mode:

5ghz WDS connection between root and extender router-
Transfer Speed on phone when connected to 5ghz WiFi of extender router = 12MBytes/sec
Transfer Speed on phone when connected to 2.4ghz WiFi of extender router = 6MBytes/sec

2.4ghz WDS connection between root and extender router-
Transfer Speed on phone when connected to 5ghz WiFi of extender router = 15MBytes/sec
Transfer Speed on phone when connected to 2.4ghz WiFi of extender router = 3.9MBytes/sec

  @terziyski Ok. I have set WDS on child router only with WDS mode 2 but its not working. Only WDS mode 1 is working. WDS 2 Is buggy in this router. But I need WDS 2 because it supports ipv6 traffic and its true layer 2 WDS with four address format and full transparent layer 2 traffic. Dlink model 825 and even much older model TL-WR841N has Range Extender feature which fully transmit all ipv6 traffic and even connects itself to internet for auto NTP time sync. But this C6 V2 is twice the price and don't have proper range extension feature :(

  @terziyski Got it. I'll check specs in detail next time before buying. I'll specifically look for range extender mode in specs page.

I recommend Tp-Link c50 (v6 or above) or D-Link 825 if anyone else looking for good dual band router with range extender mode

  @terziyski Yes it has. Many thanks. Cheers