The rapid expansion of 5G networks combined with IoT technology introduces transformative capabilities for connectivity, yet it also exposes networks to critical cybersecurity challenges. This paper systematically reviews current literature to identify the primary security concerns associated with 5G-IoT integration, including threats related to authentication and authorization, data privacy, Distributed Denial of Service (DDoS) attacks, blockchain security, and quantum computing. Each of these challenges reflects the evolving landscape of 5G-IoT, with traditional methods often falling short in handling massive device connectivity, ensuring privacy under high-data flows, or scaling effectively with blockchain solutions. In response, emerging solutions highlight the role of AI-driven security, federated learning, advanced encryption, and quantum-resistant technologies as essential tools to safeguard 5G-IoT ecosystems. Through a combination of systematic analysis and real-world examples from recent studies, this review not only categorizes the critical challenges, but also evaluates proposed solutions based on effectiveness, scalability, and feasibility. Our findings contribute to ongoing cybersecurity discussions by pinpointing gaps and offering insights into future-proofing 5G-IoT infrastructures against an increasingly complex threat environment.