The Developer Portal is available with sandbox APIs and live data.
In Sandbox you can call all sandbox APIs. And not only the ones you are allowed to call according to your eIDAS certificate.
For live data in Production, you can only access APIs that your eIDAS certificate allows you to.
Bankdata provides you with sandbox credentials, callback URI options and a sandbox certificate.
You can neither use your own test certificate nor your production certificate for sandbox.
Bankdata follows the Berlin Group Specifications on how to implement PSD2 APIs.
On the main page click [Sign up for PSD2 sandbox] or click [Sign up for API live data].