Hi @Toback Welcome to the community, as the supplied TP-LInk device is a combined modem router you need to connect your cable from your detached office to a LAN port on the gateway not the WAN port. You then only need an Ethernet switch in your office if you wish to share the cabled connection, if you only have one device there you can skip the Ethernet switch, your printer should be able to connect wirelessly to the gateway.
You may also be able to disable the router function on your existing Linksys router and use it in your office as a switch and wireless access point only.
