By default, the EIP is dissociated from the instance every time you stop or start it.
To permanently associate them, you can add the
osc.fcu.eip.auto-attach tag to the instance, using the classic tag method (with
osc.fcu.eip.auto-attach as the Name and your EIP as the Value) or the user data.
For more information, see Configuring an Instance with User Data and Outscale Tags.