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

It depends. Since Mayotte is a French Overseas Department (DOM), it is technically part of the EU "Roam Like at Home" zone. However, many European carriers have "Fair Use Policies" that severely limit data usage in overseas territories (RUP), or they may throttle speeds. For travelers from outside the EU (UK, USA, Asia), roaming fees are extremely high. A Cellesim Mayotte eSIM guarantees you a dedicated high-speed data allowance without worrying about hidden carrier limits.