Unfortunately, I can't answer the question 100%, but I understand your concern and what it is about.
Regarding Airplay, I notice that Apple always clearly states in the corresponding support article that the devices must be on the same WLAN and not just on the same network:
https://support.apple.com/de-de...
But in a similar case, people have probably already got it to work:
https://discussions.apple.com/thread...
Have you ever tried to google exactly with your configuration, e.g. by searching for "NAS ethernet Airplay" (instead of "NAS" you can also write your device)?
Just to be on the safe side: You are in the same network range in the WLAN as in the Ethernet, so you are not using a different IP range, are you?