First post!! Not sure if details correct but brand new yesterday and updated yesterday.

virgin hub 3 set as modem ( purple light) and Deco 4 working well in home. I have a studio with an iMac at bottom of garden that is ethernetted to hub3 before I set up Deco 4, that worked perfectly thru the hub 3.  It is not connected now - haven't connected cables yet.
How do I successfully connect my iMac through Ethernet to the hub or the Deco 4 as I can't work it out?! I also have a Sonos system that was connected through an Ethernet bridge - how do I set this up too?? 

Many thanks.