Roon Controller and VPN

Hello,
A question for our network gurus - I have our core running on NUC - Because our ISP is IPV6, currently to use ARC, we need to use a VPN - which is running on the NUC - This works great for ARC -

This setup however has created one unwanted by-product. The Roon controller (not ARC, the actual controller) installed on our Android phones can’t find the Core when the VPN on the NUC is turned on. They do connect to the core when the VPN is off. I guess this makes sense sort of. However, the Controller on our wired Windows desk tops as well as WI-FI Apple I-PAD does connect to the core just fine with the NUC VPN on - because of this, I’m not sure why the phones can’t find the Core? Any ideas what’s going on here?

Thanks!
pbg