Home Modem and Router Setup

Hi all, Hope some of the expert or experience network Geekzone members can help with my questions.My home network set up currently with main VDSL2 modem router downstairs and LAN powerline up to a router upstairs. It's currently working great but recently I purchased Seagate personal cloud NAS drive.The way it set up now the main modem and router upstairs work in different domain/subnets and therefore for the NAS only visible in the main modem domain and none of the upstairs devices can't see the NAS drive.Could anyone advise provide a great link for me on:

How I can set up the router upstairs to be part of downstairs modem/router domain/subnet

Whether this is the best way to set it up without degrading the current network performance as currently it's working great

Thank you in advance :)