I have a home media server running minidlna daemon, and I can stream media to my android phone when I am connected to the home LAN, but I can't seem to be able to do it over the internet. I'm using BubbleUPNP and I've already forwarded the ports on the home router, but I don't see any option/setting on the app to change the server address/port.
This does not work because your phone does not receive the advertisement packets sent out by your media server and vice versa. Those packets are only forwarded within your home LAN but not over the internet (And for good reason: otherwise this would mean that every device connected to the internet would receive those advertisement packets).
One way to overcome this limitation would be to set up a VPN (like WireGuard, OpenVPN, IPsec, or the like). Thus your phone would get an IP address in, and stay connected to your home LAN, even if you are not on-site.