@Yvonne Arnoldus Have you double checked if you are connected to the same directory? If the user is added as guest to any other tenant, it may connect to that tenant based on your preference in portal.azure.com. You might need to switch directory and connect to the right tenant in that case.
If both Azure Portal and Azure AD Portal are connecting to same tenant, could you please share more details about the error. Are you getting any error message while trying to assign a role or if the option to assign role is grayed-out? If you capture a fiddler or browser's network capture (using F12 key), what HTTP error code you see? Did you try with a different browser or in-private/incognito mode?
As long as Azure Portal and Azure AD Portal are connecting to same tenant, you should have same functionality.
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