Why can my internal network not access any externally exposed service?

Thanks, but that's not it.
My network is as simpel as it gets. Only one LAN, and servers inside that LAN.

Example:

Phone outside house (i.e. NO WiFi) -> Internet -> UTM -> My Mail Server (works fine)
Phone inside house (i.e. WiFi) -> Internet -> UTM -> My Mail Server (doesn't work)

So, when my phone has a public IP it can get mail from the mail server, but when my Phone has an internal IP (same range as the mail server), it cannot get mail.