@Dan1987 

In Deco world, manual configuration of how Deco units should connect to each other is called signal source selection. That feature is described here: How to select a preferred signal source on the Deco App

It is only available for small subset of all Deco models, and S7 is not on that list. See full list of Deco models with that feature available:  https://community.tp-link.com/us/home/forum/topic/597476

The only other options for S7 I can think of:

1. Move furthest Deco even further away from Main Deco and see if that forces it to connect to in between Satellite Deco;

2. Connect you two Satellite Deco with Ethernet cable;

3. Connect you two Satellite Deco through powerline adapters.