Hello Diane,
Regarding the cable modem’s second ethernet port the SB8200 is a MODEM, not a router The second ethernet port allows this modem to support “speeds” above 1 gigabit per second (Gbps). To take advantage of this you need a router with a feature called port aggregation.
If you are not paying for internet faster than 1 Gigabit per second, then disregard the second port on the cable modem.
You will want to plug your computer into the eero device’s second ethernet port.