Ok, how it should work if all things are good...I am from USA/Canada region, so I am in that perspective.
1. Your router's DMZ is set to ".98" at the end (example 192.168.1.98).
2. The router's IP range allows for it to work at ".98". I'm Canada/USA, so I cannot speak of your region if outside North America.
3. You boot the DreamPi and once it's done booting up, it is ready to go...no touching commands.
4. Both USB modem lights are on.
5. The Dreamcast game disc is loaded and the DC is booted up after being set up via XDP and sometimes, you might have to use Q3 to finish the set-up correctly. Out of 7 of Dreamcasts I have set up, for some reason, two of them I had to use Q3 to finish the job...I have no explanation beyond that. Plus three of them, I had to use DC Flash Eraser for some weird PSO related reason.
6. Dialing.....connecting....logging in - if required....gaming.
1. Please check things in that sequence, but making sure the configuring the Dreamcast is indeed correct prior to anything else.
2. Redo the DreamPi SD card and do not touch it (unless where you live, you have to for other purposes).
3. From there, all things should work as I stated above. If not, please advise, I will do my best to help you with this issue.