Draytek manual only talk about creating 2nd subnet for public IP addresses.

However, I have created 2nd subnet (private subnet) for IP routing usage.

first subnet 192.168.1.0
second subnet 192.168.2.0

Traffic from 1st subnet passes thorugh to 2nd subnet but traffic from 2nd subnet to 1st subnet get blocked.

On routing table it shows 1st subnet "C~" i.e connected and private.
2nd subnet "C" connected.

Need help.

The 2nd subnet feature is specifically for multiple public IP addresses, the router can't have more than one LAN address.
If you want the router to access both 192.168.1.0 and 192.168.2.0, you'd need to change the subnet mask.