Is roaming free in Belarus with my European (EU) or UK SIM card?

No! This is a major roaming trap. Although Belarus is in Europe, it is NOT a member of the European Union (EU). Your "Roam Like at Home" (RLAH) plan does not apply. Most EU and UK carriers (like Vodafone, O2, T-Mobile DE) will charge you extremely high "Rest of World" roaming fees (e.g., £6-£10 per day). A Ti Porto in Viaggio eSIM is the only way to get a fixed, cheap data price.