There is a tool that you can use, to wipe the flash on the Dreamcast itself. IF you want to try that,
here's the link. BUT! Please make sure you read everything on this page, and act accordingly. Because, if you don't, you might end up with an official Sega Dreamcast paperweight.
However, I doubt that'll fix the issue. But it's cheaper than buying a whole other console, eh? lol. Either way, it's up to you. Let us know what works for you. It also might be worth reaching out to anyone else you know, who also has a compatible Dreamcast, and give it a try on their systems.
Also, I've seen some stuff in the "Dreamcast Talk" Discord regarding issues connecting. In some cases, it required them to call up the ISP directly to allow it through. It might be worth looking into. It should be in the "Connection Help" section. Regardless, it might be a good idea in general to just join the discord, as you'll most likely get a response much more quickly.