Hi kayut

It largely depends on the modem, if it is running a router or the like. If the modem is running a router yes, you should be able to plug the gateway/head eero into any ethernet port on the modem (topology requirements still apply: https://support.eero.com/hc/en-us/articles/360000830546-Examples-of-common-network-topologies ).

However, if the modem is not running a router or if the eero is setup on the modem's DMZ you likely don't want to change the port, it may work, it may not.

Of course, you can always change the port and if it doesn't work switch it back.