A network access control (NAC) appliance is a device that checks the enrollment and compliance state of devices that try to access the network resources. It can deny, quarantine, or restrict the access of non-compliant devices based on predefined policies1. A device can be considered non-compliant if it does not meet the security requirements, such as having the latest patches, antivirus signatures, firewall settings, or encryption standards. In this scenario, the user's laptop may have been labeled as non-compliant by the NAC appliance because it was out of sync with the network policies after working remotely for a long time. The user connected to the guest network, which is usually less secure and isolated from the corporate network, and updated the patches and software, but that may not be enough to satisfy the NAC appliance. The user may need to enroll the device again, or contact the IT support to resolve the issue.
References1 - Network access control integration with Microsoft Intune | Microsoft Learn