@Maegllin 

So, you manually set a new LAN IP address on the VR400 that is in the same subnet as the ASUS router. For example, if the ASUS router's LAN IP address was 192.168.1.1, then your VR400's LAN IP address might be set to 192.168.1.2 (or something else in the pattern 192.168.1.x), but if you try to open http://192.168.1.2 in a browser, then it won't open the VR400's UI.

Can you connect your computer to one of the LAN ports of the VR400 via Ethernet cable and try again?

  @woozle

I've tried also connecting VR400 through cable with a laptop but typed the ip (let's say: http://192.168.1.2) and didn't work also. I can't get with any way possible in VR400's UI

  @Maegllin 

Can you log into the ASUS router's UI and see if you can find the VR400 in the list of connected devices? (maybe under "Network Map")

In case the VR400 does show up in the list of connected devices on the ASUS router, then check whether the IP address associated with the VR400 is indeed the one you have manually set or not.

  @woozle 

There is totally no VR400 inside the list of connected devices of "Network Map" except the devices which are connected to the VR400 but not VR400 itself.

Where I can check about VR400's IP that I've set manually inside ASUS's interface?

  @Maegllin 

If it doesn't show under "Network Map, then I guess you can't.

You could use some network scanner software to scan the IP range of your subnet for devices.

"Angry IP Scanner" would be a free one, but there are many others as well.

In case you do, it would be best to have as few devices connected to your network as possible at the time of the scan, to reduce the number of hits and hence make it easier to determine which on is the VR400.