How do I get these Internet/Wi-Fi details to give to tech support and is it safe? I have been having Internet connection issues for weeks and my ISP sent me a new Modem/Router today, but the issues are still there. The tech has opened up an online case I can respond to and attach screenshots if need be.

Now the tech is asking me for the RSSI, Noise and TX Rate values.

How do I get these details? And are they safe to give to him?

I’m using an iMac with macOS High Sierra installed.
Yes, those details are safe to provide. They are measurements of the quality of your wireless signal, and they contain no personal information.

You can view these and other details from the Wi-Fi icon in the menu bar. The trick is to press and hold the option key while you click on the icon (i.e. make sure you’re pressing the option key first before clicking on the menu bar). If you do you’ll see these values and more. 

[UPDATE]

Prompted by Gordon Davisson's comment, this update adds more information about the options available under the Wi-Fi menu.

As mentioned previously, by holding down the option key while you click on the Wi-Fi icon you'll be able to see the values you were after and more. Some additional options include:


Enable Wi-Fi Logging
Create Diagnostics Report...
Open Wireless Diagnostics...


For a broad overview of these options you can refer to Check for Wi-Fi issues using your Mac.

In summary, however, selecting the Open Wireless Diagnostics... option will launch an application that helps users to diagnose common issues with their Wi-Fi connection. Note: This refers to your Wi-Fi connection and not your internet connection per se, although not having internet access on a device could be in fact related to your Wi-Fi connection.

While you can run through the Assistant that opens when you first launch Wireless Diagnostics, you can choose to ignore this and manually access additional utilities available under the Window menu (or by using the relevant optioncommand shortcut). One of the best things about this is you can actually open all of them simultaneously if you want to.

By using the Performance utility you'll get graphs of those values over time (along with "Quality", which is just the difference between RSSI and Noise). You can use these graphs to gauge how they change as a result of certain actions (e.g. moving your computer, checking to see how Noise reacts when using a microwave or cordless phone, etc). This is particularly handy to keep open so you can monitor it if you're having problems.