The output is most likely an example of an ARP poisoning attack. ARP poisoning, also known as ARP spoofing, is a type of attack that exploits the ARP protocol to associate a malicious device's MAC address with a legitimate IP address on a local area network. This allows the attacker to intercept, modify, or redirect network traffic between two devices without their knowledge. The output shows that there are multiple entries for the same IP address (192.168.1.1) with different MAC addresses in the ARP cache of the device. This indicates that an attacker has sent fake ARP replies to trick the device into believing that its MAC address is associated with the IP address of another device (such as the default gateway). Reference: https://www.cisco.com/c/en/us/about/security-center/arp-spoofing.html

Change management is an IT practice that aims to minimize disruptions to IT services while making changes to critical systems and services12.
A change is defined as adding, modifying, or removing anything that could have a direct or indirect effect on services1.
Change management involves planning, testing, communicating, and implementing changes in a controlled and coordinated manner2.
Change management can help prevent network outages and other incidents by ensuring that changes are properly assessed, approved, and documented before they are executed2.
Incident response, on the other hand, is the process of detecting and responding to security incidents, such as cyberattacks or data breaches3.
Incident response focuses on the technical aspects of analyzing and containing an incident, while incident management deals with the broader impact of an incident on the business, such as communications, media handling, escalations, and reporting3.
Standard operating procedure (SOP) is a set of instructions that describes how to perform a routine or repetitive task.
SOPs are useful for ensuring consistency, quality, and compliance in various operations, but they are not specific to change management.
System life cycle is a model that describes the phases of development, operation, and maintenance of a system, such as a software application or a hardware device.
System life cycle can help guide the design, testing, deployment, and improvement of a system, but it does not address the issues of change management, such as minimizing disruptions and risks.
Therefore, the best document for the network manager to create to prevent future network outages caused by firmware upgrades is a change management policy.
Reference:
Change management types | Atlassian
Incident Management vs. Incident Response - What's the Difference? - DZone
10 Top change management best practices | Atlassian
[What is a Standard Operating Procedure (SOP) and How to Write It]

Explanation
The best option for the administrator to take would be to attempt to connect to the site via IP address. This will help to determine if the issue is related to the website's DNS address or if the site itself is not accessible.
Checking for certificate issues may be necessary, but this should be done after the administrator has attempted to connect to the site via IP address. Contacting the ISP is unnecessary since the issue is related to the website inside of the company network, and checking the NTP configuration is not relevant to this issue.
When a customer is having issues connecting to a website using a URL, one of the first troubleshooting steps a network administrator should take is attempting to connect to the site using the IP address of the website. This will help to determine if the issue is related to a DNS resolution problem or a connectivity problem. If the administrator is able to connect to the website using the IP address, then the issue may be related to a DNS problem. However, if the administrator is still unable to connect, then the issue may be related to a connectivity problem. In either case, further troubleshooting steps will be necessary. Checking for certificate issues or NTP configuration, and contacting the ISP would not be the BEST initial steps in this scenario.