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.

A heat map would most likely be created as a result of the site survey. A heat map is a graphical representation of the wireless signal strength and coverage in a given area. It can show the location of APs, antennas, walls, obstacles, interference sources, and dead zones. It can help with planning, optimizing, and troubleshooting wireless networks. Reference: https://www.netspotapp.com/what-is-a-wifi-heatmap.html

See the solution configuration below in Explanation
Explanation:

The signal-to-noise ratio (SNR) is the difference between the received wireless signal and the noise floor, which is the level of background noise on the spectrum. A high SNR means a good signal quality, while a low SNR means a poor signal quality. The noise floor can be affected by various sources of interference, such as cordless phones, microwaves, radar, etc. To troubleshoot a wireless connection issue caused by a low SNR, the technician should first identify and remove any electronics that might be causing interference in the area. This would improve the SNR and the wireless performance. Running diagnostics on the relevant devices, moving the access point to a different location, or escalating the issue to the vendor's support team are possible steps to take after removing the interference sources, but not the next step. Reference:
Signal-to-Noise Ratio (SNR) and Wireless Signal Strength
Wireless: SNR, RSSI and Noise basics of wireless troubleshooting
What is Signal-to-Noise Ratio and How to Measure it
Network + N10-008 practice exam
CompTIA N10-008 Free Certification Exam Material
Free Practice Questions for CompTIA Network+Exam (N10-008)

The OSI model is a conceptual framework that divides the network communication process into seven layers, each with a specific function and protocol4.
The session layer is the fifth layer of the OSI model, and it is responsible for establishing, maintaining, and terminating sessions between applications on different devices45.
A session is a logical connection that allows two or more applications to exchange data for a certain period of time5.
For example, when two users on a LAN establish a video call, the session layer coordinates the initiation, synchronization, and termination of the call, using protocols such as SIP (Session Initiation Protocol) or H.32356.
The session layer also provides services such as authentication, authorization, encryption, and checkpointing5.
The other layers of the OSI model have different roles in the network communication process, such as:
The physical layer is the first layer, and it is responsible for transmitting and receiving bits over a physical medium, such as a cable or a wireless signal4.
The transport layer is the fourth layer, and it is responsible for ensuring reliable and error-free data transfer between devices, using protocols such as TCP (Transmission Control Protocol) or UDP (User Datagram Protocol)4.
The data link layer is the second layer, and it is responsible for framing and addressing data packets over a local network, using protocols such as Ethernet or Wi-Fi4. Reference:
4: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 1: Introduction to Networks, pp. 22-29
5: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 11: Network Services, pp. 443-444
6: CompTIA Network+ N10-008 Cert Guide, 1st Edition, Chapter 11: Network Services, p. 452