@Rahmat.A 

There is no harm in trying. Just follow what is said on page 5, make sure you power off Deco and old router before disconnecting them, and power on Deco after it is connected to modem.

If everything works, keep it. If Deco does not work in that configuration - undo changes by connecting old router back to modem and deco to old router.

If you had to undo changes and need to keep old router, login to it and turn off its WiFi. 

@Alexandre. 

Ok will try..thnks for reply

@Rahmat.A 

I can also explain logic behind guidelines.

For initial Deco setup, you need to connect Deco to working home network. Your unifi router provides working home network, so this is where Deco must be connected.

After initial setup, Deco by itself can provide working home network, making unifi router unnecessary. This is when you can remove old router and connect Deco directly to modem.

Of course, unifi router still can provide working home network, so if after connecting Deco directly to modem something does not work, it is safe to undo changes to home network by bringing unifi router back online and connecting it to modem. Your home network should start working again after that.

I am assuming it is your Internet provider that has given you Deco and guidelines manual. If this is correct, you can contact your Internet Provider Tech Support if following manual didn't work and you had to bring unifi router back online and reconnect it to modem.