I saw on Tor's FAQ that a Tor Relay can work well with a dynamic IP, I was wondering if it's the same for a service or if it would be sometimes unreachable ?
TLDR; It is the same for a service, as long as your server is connected to the tor network.
To make it short, when you set up your hidden service, it will pick 3 (by default) introduction relays, and add their addresses to your service description. Whenever your server is connected to the tor network, it will establish a connection to its introduction relays. Then, when a client wants to reach your service, he will forward you an invitation using your introduction points. Therefore, it does not depend on your IP address.
The only scenario where your service might be unreachable, is if all your introduction relays are down too. But you can have up to 10 (v2) or 20 (v3) introduction points, depending on your hidden service version.