Reicast Libretro has now gained modem and PPP support, which uses dreamcastlive.net to make online play possible. This new update should work flawlessly on Windows, Linux, Android and even macOS without inputting gibberish like IP address. What is that anyway?(I’m being sarcastic)
According to Libretro, this is the first time a emulator has supported a zero-configuration online gaming for Dreamcast and since it uses dreamcastlive.net, it seems you will be able to play with actual Dreamcast servers. What a good timeline. More information here.