Lars The endpoint is briefly mentioned here: https://beta.doc.bunq.com/basics/authentication#changing-allowed-ip-addresses-after-initializing-api-key
It is possible to add trusted IP addresses using the IP endpoint. Obviously you need to make that request from an IP address that was allowed previously.
In general: you can use POST to add a new entry and PUT to modify an existing entry. In the case of this endpoint I would use the POST method to add a new IP and set it as active, and then if you want to retire and old server use PUT to set the corresponding entry to inactive so that it cannot be used anymore. Personally I haven't implemented this in my own app, because I work on a very small scale with just a single server.
But I would suspect the "credential-password-ipID" is probably the one you can get from the credential-password-ip endpoint. If you try out some calls with both of the endpoints in sandbox it should be quick (and safe) to figure it out. And if someone from bunq reads this here: the documentation could make it a bit more explicit how to do this! Thanks :)