iPhone/iPad connected via VPN to Core on QNAP

When I connect using my iPhone/iPad via VPN to my core on a QNAP NAS then in most cases I cannot use the devices as audio zone. Sometimes- after restarting the app- it appears. Is there a way to improve this? I am using both on Nas and the devices the current Roon version.

Meanwhile i am using an iPhone 12 and an iPad Pro (11inch, 2021) with the latest IOS Version 14.6. The connection to Roon core via VPN works well - but I cannot use the iPhone/iPad as audio device. Are there any ideas how to resolve this?