Nextcloud Talk Mobile app not receiving call notifications

I have installed Nextcloud in a private network and connected the mobile app to my Nextcloud instance after connecting via VPN to my private network. Unfortunately when I receive an incoming call, I do not see any notifications. If I am in the chat window with the user I do see that they are trying to call me.
I suspect that the notifications do not work because Nextcloud Talk uses Apple’s ASPN, which cannot connect to my phone. If someone can confirm, if this is indeed the case I would appreciate it. Thanks in advance.