One of the best practices to secure the network would be to disable unneeded switchports in the guest waiting area. This will prevent unauthorized users from connecting to the network through these ports. It's important to identify which switchports are not in use and disable them, as this will prevent unauthorized access to the network.
Other practices such as ensuring all guests sign an NDA, lowering the radio strength to reduce Wi-Fi coverage in the waiting area and enabling MAC filtering to block unknown hardware addresses are not as effective in securing the network as disabling unneeded switchports. Enforcing an NDA with guests may not stop a malicious user from attempting to access the network, reducing the radio strength only limits the Wi-Fi coverage, and MAC filtering can be easily bypassed by hackers.

First-hop redundancy protocols (FHRPs) work by giving you a way to configure more than one physical router to appear as if they were only a single logical one.

Asymmetrical routing is the most likely cause for the WAN instability. When two different routing protocols are used, like OSPF and EIGRP, it can cause asymmetrical routing, which results in traffic being routed differently in each direction. This can lead to instability in the WAN. A CDP neighbor change, a switching loop, or an incorrect IP address are not likely causes for WAN instability.

A VLAN (Virtual Local Area Network) is a logical grouping of network devices that share the same broadcast domain, regardless of their physical location or connection1.
VLANs can be used to segment the network into different departments, functions, or security levels, and to improve the performance, security, and manageability of the network1.
To access the resources on a VLAN, a device needs to be assigned to the same VLAN ID, either statically or dynamically, and the switch needs to be configured to support VLAN tagging and trunking12.
In this scenario, the most likely cause of the problem is that the last few salespeople were assigned to the wrong VLAN, either by mistake or by lack of available ports on the correct VLAN3.
This would explain why they can access the corporate materials on the network, which are probably shared across all VLANs, but not the sales-specific resources, which are probably restricted to the sales VLAN3.
The other options are less likely to cause the problem, as they would result in different symptoms or errors, such as:
The switch was configured with port security: Port security is a feature that limits the number of MAC addresses that can be learned on a switch port, and prevents unauthorized devices from accessing the network. If port security was the cause, the new devices would not be able to connect to the network at all, or they would trigger a security violation and shut down the port.
Newly added machines are running into DHCP conflicts: DHCP (Dynamic Host Configuration Protocol) is a protocol that assigns IP addresses and other network configuration parameters to devices automatically. A DHCP conflict occurs when two devices have the same IP address on the network, which causes connectivity issues and IP address errors. If DHCP conflicts were the cause, the new devices would not be able to access any network resources, not just the sales-specific ones.
The IPS was not configured to recognize the new users: IPS (Intrusion Prevention System) is a device or a software that monitors the network traffic and blocks or alerts on any malicious or suspicious activity. If the IPS was not configured to recognize the new users, it might flag their traffic as anomalous or unauthorized, and block or drop their packets. However, this would affect their access to all network resources, not just the sales-specific ones, and it would also generate IPS logs or alerts that could be checked by the network administrator. Reference:
1: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 7: Virtual Networking, pp. 279-282
2: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 7: Virtual Networking, pp. 283-287
3: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 7: Virtual Networking, p. 288
4: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 14: Network Security, pp. 581-582
5: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 11: Network Services, pp. 447-449
6: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 14: Network Security, pp. 589-590

Explanation
The logging level debugging is the highest level of logging, which means that the switch will log every possible event, including low-priority and verbose messages. This can result in excessive amounts of data being sent to the syslog server, which can affect the performance and storage of the server. To fix the issue, the technician should adjust the logging level to a lower value, such as informational, warning, or error, depending on the desired level of detail and severity. This will reduce the amount of log data generated by the switch and only send the relevant and necessary messages to the syslog server.
https://betterstack.com/community/guides/logging/log-levels-explained/