I have two network with two different ISP. The first network (let say Network A) consist of PCs, Laptops, Printer and NAS. The second network (let say Network B) consist of PCs and Laptops. What I want to do is to join both of the network, so users in Network B can access NAS and printer in Network A. Both of the network have different ISP, and each network should only use their corresponding ISP. Both of the network connected to Asus RT-N66U for wireless devices. So how to connect this two network without each network using other ISP?
Is there actually a cable running between your two Asus routers? If there is, you don't need to go through the trouble of setting up a VPN between the two devices.
The whole point of a router is connecting disjointed networks. That is literally its only job. We've got used to seeing multiple devices bundled together as "routers", but what you've really got there is a router/firewall/access point/general purpose operating system. We just need to focus on the routing part of it.
I don't know how complicated the operating system on those Asus RT's are, but what you would normally do here is create a tiny network just for the cross-router links, and then tell the router to use that link for accessing the other links.
For example, you would give the router on the left an additional IP address of, say,
192.168.30.1/31 and the router on the right an IP of
Then you would tell the left router that it can access
192.168.30.2, and you would tell your right router to access
The specifics of exactly how to do this I leave to you, as I am not familiar with that device, and from what I can see there are a variety of different operating systems it could be running.