@DaveinNE 

Yes the other Ethernet port on the Deco S4 will work to get your Dell on the network. 

  @DaveinNE By the way, " My modem only has one Ethernet port", is a good thing.

That forces any network connectivity, Ethernet or Wi-Fi, to be in the Deco network.

That avoids the potential for accidental sub-netting devices into the ISP's IP range.

Which would cause network connectivity issues, between your Deco networked devices and any devices connected directly to the Modem, as they would not be in the same IP range.