If you're using a homemade LVI circuit you soldered up (odds are you will need an LVI between the Dreamcast and DreamPi), I'd double check if the solder connections are good. A solder joint broke off when I was testing my wired DreamPi setup for the first time, and it took like a day to troubleshoot that as the cause of the modem connection issues. The modem connection would be flaky or not work at all if the LVI solder joints are flaky or disconnected.
There is also a NOOBS compatible DreamPi image
(v1.6) if you wish to boot multiple OSes and DreamPi on the same SD Card while troubleshooting, with PINN being recommended to use.