Hi @Arty29 - before you can use the SPN with IoT Central you must assign the SPN a role within IoT Central (the Azure Roles do not apply to IoT Central since it's a SaaS product with different capabilities).
Today the only way to add an SPN as a 'user' in IoT Central is through the API surface (we plan to add it to the UI in the future). You can see documentation here with examples on how to add the SPN to your app: https://video2.skills-academy.com/en-us/rest/api/iotcentral/users/set#add-or-update-a-service-principal-user.
You can learn about IoT Central roles here: https://video2.skills-academy.com/en-us/azure/iot-central/core/howto-manage-users-roles#manage-roles , the role ID you'll need to input as part of the API GET call can be found in the address bar when you navigate inside the Role details.
Hope that helps! Let us know if that worked for you.
-Luis