Hello, and thank you for your feedback.
We received an answer from our developers about this current issue, and they said that unfortunately there is no way to configure our hardware at a bit level to handle Ethernet frames. It's hardware dependent, so, some hardware will handle it while others won't.
Answering your question, the RUT240 interfaces logically work as a switch between it's two interfaces (when it's enabled for WAN to LAN), however, we cannot confirm or deny it is going to work the scenario you are describing as it is specific for your equipment.
For this case, if you need a Switch, you can check our device TSW100, which is a Gigabit switch with five ports, and four of them are PoE.
I'll keep an eye out for any following answers on this issue.
Best regards.