If this was a feature they wanted to provide, it would simplest to set the Roku up as a bridge. No dhcp, no nat, just a virtual switch. Roku devices run on Linux, if their HDMI chip supports Ethernet and that's plumbed to a Linux supported MAC (probably not), it might take an hour to build and test the feature. Longer if the wifi interface doesn't like being bridged (I've seen that on some openwrt devices).

There's esp-idf examples from espressif for doing bridging on an esp32 if you have one with a MAC and a PHY and/or spi mac+phy.

Or for wired ethernet, they could include a three port switch IC.