I have a Spectrum router also and bought the Eero system as my Spectrum wifi box would not work.
The Eero system has one 'base' unit and two remote beacons. The Base unit should be placed by your router and plugs into it via one of the two Ethernet ports on the back of the Spectrum router. Then you download the eero app and follow the instructions. The remote beacons need to get placed within 50 feet of the base unit to give good coverage, but the app guides you through that process too and tells you if you have them placed in good positions. I have a 3,100 square foot house on one level and needed to buy one extra beacon (three total plus the base unit), to get full uninterrupted coverage inside the house and in the back yard too.