As Mobile Network Operators (MNOs) prepare to interconnect diverse technologies to existing cellular networks, it is critical to assess the capabilities of the network architecture to handle such changes. One key area to consider is the subscription management process, which governs the user's profile management, authentication, and access control. This process operates centrally, which affects users' security, accessibility, and privacy. Additionally, it increases the system's complexity when handling the large volume of messages sent over networks like IoT. In this work, we propose a Blockchain-based subscription management approach for next generation cellular networks to address the challenges in user profile management and the Authentication and Key Agreement (AKA) process. The method uses a hybrid cryptosystem technique to protect the user's privacy. Based on the evaluation, the system can handle the AKA process with fewer messages passing while improving system availability by utilizing distributed network functions and storage. Finally, we highlight some key points to consider when implementing our proposed approach.