If you have region issues, some Flycast builds allow forcing region: Settings → Dreamcast → Region (Japan / USA / Europe).
Emulators are highly sensitive to file naming conventions. If your files are named incorrectly, Flycast will fail to recognize them and default back to its internal HLE BIOS. Dreamcast Bios Flycast
Here’s a on the Dreamcast BIOS in the context of Flycast (a popular standalone Dreamcast emulator, also a core in RetroArch). If you have region issues, some Flycast builds