I am helping my neighbor configure his 3 node TP-Link Deco M5 mesh network in a large house with structural masonry walls that prevent full WiFi coverage from the T-Mobile's Gateway WiFi.  The Deco mesh topology is intended to ultimately be in Router mode using Ethernet Backhaul to connect the gateway and at least the first and second Deco mesh nodes.

First off, I had configuration problems when placing the first Deco node in its finished location about 30 feet away from the T-Mobile gateway through a 40-foot Cat 6 cable.  However, when I put the base Deco mesh node next to the gateway and connected with a shorter cable, the node configuration was successful.  After configuration, I moved the base node back to its ultimate location and configuration of downstream nodes failed.

Do all of the Deco mesh nodes need to be initially configured in close proximity to the gateway, then moved to their distant locations and cable-connected?  If the configuration procedure uses bluetooth, this might require the closer proximity, at least for the initiial configuration.

Can I configure and install just Deco mesh node1, or nodes 1 and 2 which will be cable-connected, and leave mesh node 3 for later installation?  Or do I need to configure all 3 nodes at once in close proximity to the gateway?

Thanks for any advice.