Is roaming free in French Guiana with my European (EU) SIM card?

Be very careful! As French Guiana is an overseas department of France, it is technically part of the EU, and "Roam Like at Home" (RLAH) should apply for EU citizens. However, most non-French EU carriers (e.g., from Germany, Spain, Italy) enforce strict "Fair Use Policies" for overseas territories, which can lead to high fees or throttled speeds. For UK (post-Brexit) or US travelers, it is definitely expensive international roaming. A Ti Porto in Viaggio eSIM guarantees a fixed price and high-speed data.