Is it possible to add my DDWRT(VPN) router with my main ISP modem/Router ? Follow
Yes it is .
You can add as many routers as you can by using DHCP mode enabled in your new DDWRT.
However, You need to change your IP range in DDWRT router.
e.g. if your main ISP router is using ip range 192.168.1.1 so you can set your DDWRT router with 192.168.2.1.