@Hasayeret 

If you have modem which is separate from the router, I would recommend connecting Main Deco directly to the modem. Main Deco in its default Router mode runs as a standard router with all commonly used router features, so you are not gaining anything by placing Archer VR400 between Main Deco and modem.

You will also simplify your home network by not having Archer VR400 between modem and Main Deco.

  @Alexandre. - thanks!

few more questions assuming I'm going with your path:

1. is there anything in particular I need to do in the setup to define my mesh as a router?

2. do I need to enter any details to the mesh or will it immediately grab what it needs from the modem?

thanks!

  @Hasayeret 

1. You will not need to do anything to define Deco mesh as running in Router mode. 

2. You should have Quick Start guide with Deco set, which will guide through initial setup and installation. That Quick Guide should be similar to the following document: Deco Setup Walkthrough(For Deco X20/X60/X90/P9/M4/S4/E4/W2400 etc)

  @Alexandre. - thank you!

someone told me that I should either configure it as pppoe or l2tp - how should I know which one I should choose and once I know, how can I configure it?

  @Hasayeret 

If you have Archer VR400 connected to the modem, check its connection settings and use same for Main Deco.

  @Alexandre. But I'm moving to a brand new place with a brand new modem. The router isn't going to be there.

any idea?

  @Hasayeret 

Ask your ISP at new place for configuration settings, how to configure your WiFi router to connect to their modem. 

If you can take Archer VR400 to new place at least temporarily, it may be easier for you to use these setting on Archer and see if they work. When they do, try same with Deco. If they don't, ask ISP Tech Support for help. It'll be easier for them to direct you how to configure Archer VR400 than how to setup Deco mesh.

  @Alexandre. 

Got it. 
and there's no point keeping the Archer in place along with the Deco?

  @Hasayeret 

I can think of only one reason to keep Archer router. If you can configure at new place Archer to connect to modem, but fail to do that with Main Deco. In that case, keep Archer connected to modem and Internet, connect Main Deco to Archer and configure Main Deco connection type as "Dynamic IP." This should work.

This setup, while not optimal and usually not necessary, will get you functioning WiFi mesh and Internet connection. With that, you'll have all the time you need to figure how to bypass Archer and have Main Deco connected to modem.

  @Alexandre. 
thanks so much!

will the installation prompt me to choose a dynamic IP address when I go the way you suggested?

also, I assume that if/when the deco will go directly to the modem it'll be a static address, correct?