DIAGNOSIS Yes, you can, but it doesn't work like a simple switch out of the box. The Deco's Ethernet ports are primarily for connecting the Deco to your modem (Internet port) or for giving a wired connection to a device (like a PC or game console). For a printer to be seen by other devices, it needs an IP address from the Deco's DHCP server, which should happen automatically.
FIX First, get the physical connection right. 1) Use an Ethernet cable to connect your printer to one of the two LAN ports on any Deco unit (not the one labeled 'Internet'). 2) Power cycle the printer. Turn it off, wait 30 seconds, and turn it back on with the Ethernet cable connected. It should get an IP address. 3) On a computer connected to the Deco WiFi, you need to find the printer's new IP address. You can often print a network configuration page from the printer's own menu. Alternatively, open the Deco app, go to the 'Clients' tab, and look for your printer's name or MAC address in the list.
IF THAT DIDN'T WORK The printer might be set to a static IP that conflicts with your network. You'll need to use its control panel menu to set it to 'DHCP' or 'Auto IP'. Consult your printer's manual for how to do that. If it's an older printer, it might only support 100 Mbps Ethernet. That's fine, the Deco port is backwards compatible.
HEADS UP You can't plug an *unmanaged* network switch into a Deco port and then plug multiple devices (like a printer and a NAS) into that switch without potentially causing network loop or performance issues. For multiple wired devices, you should use a switch connected to your modem (if it has ports) or consider a different network setup.