Hi @Jesús Martín Adrados,
Welcome to the Microsoft Q&A platform!
Exchange Online Plan 1 and Plan 2 do not include Exchange Server Client Access Licenses (CALs) for on-premises Exchange Server use. This means that if you need to access an on-premises Exchange Server, you would need to purchase the appropriate Exchange Server CALs separately.
Please feel free to contact me for any updates. And if this helps, don't forget to mark it as an answer.
Best,
Jake Zhang