By default, the EIP is dissociated from the instance every time you stop or start it.
For a persistent association, you can add the
osc.fcu.eip.auto-attach tag to the instance, using a regular tag (with
osc.fcu.eip.auto-attach as Key and your EIP as Value) or the user data.
For more information, see Configuring an Instance with User Data and 3DS OUTSCALE Tags.