Unable to make the Mullvad Android app connecting

[hushcoden]

I'm testing the Mullvad app on my smartphone (it uses Wireguard) and it works just fine.

If I connect the smartphone to my home network, then OPNsense denies the connection (see attachment): where do I start to troubleshoot?

On the same smartphone, I have also ProtonVPN app and it connects straightaway 

Tia.