5G technology has revolutionized the IoT landscape by providing faster, more reliable, and scalable connectivity. Notably, the integration of Massive Machine-Type Communication (mMTC) applications in Internet of Things (IoT) has enabled seamless connectivity for a multitude of devices. However, this proliferation of IoT devices has also brought forth a host of security challenges. Some of the primary security issues in IoT encompass device authentication, data privacy, and different attacks. To address these challenges, various algorithms and techniques are being employed, including advanced encryption methods, blockchain technology, cryptography, public key infrastructure security mechanisms, and anomaly detection using machine learning and deep learning. These measures aim to fortify the security posture of IoT ecosystems, ensuring the confidentiality, integrity, and availability of data transmitted through 5G networks. Furthermore, the integration of Software Defined Networking (SDN) plays a pivotal role in addressing these security issues. By harnessing the combined potential of SDN, along with threat detection and additional security measures, the overall security robustness of 5G-networked IoT ecosystems experiences a significant enhancement. This article offers an extensive review of current research, centering on three key areas. To begin with, it encompasses a diverse array of applications connected to mMTC within the IoT. Secondly, it investigates the manifold security and privacy issues associated with network technologies in the IoT environment. Lastly, it examines the pivotal role that SDN plays in bolstering IoT security.