User with an account created under Reseller cannot log in to CCP under Reseller's brand but can do this into Provider's one. Test user can sign in normally to CCP under Reseller's brand.
At the moment of the user creation, Reseller did not have an own brand. That is why the Provider's brand was assigned to the customer's account. Now Reseller has their own brand and the newly created test account has been assigned to the Reseller's brand. As a consequence, the newly created account has access to CCP under the Reseller.
There is no way to change a brand assigned to account via Control Panel. Preferably re-create the user's account. If it is not an option, please contact Odin Technical Support in order to proceed with Management Node database intervention.