Preview on devices QR not showing in dreamweaver

Unfortunately, it's not just unreliable, it has been discontinued by Adobe: https://helpx.adobe.com/dreamweaver/using/device-preview-troubleshooting.html#:~:text=Effective%20from%20November%2016,%202021,the%20pages%20on%20multiple%20browsers. 

Setting up a testing server (and running it as administrator) is the best way to preview across any device on your local network. Once set up, all you need to do is run a quick ipconfig in a Command Prompt to find your computer's local network IP, then go to that IP followed by the root folder of your site's name to view it on any device connected to your network. For example: 123.45.67.89/MyWebsite would go in the address bar of your phone's browser.

Setting up testing server is pretty simple and free: https://helpx.adobe.com/dreamweaver/using/set-testing-server.html

I use XAMPP: https://www.apachefriends.org 

Getting your local network IP: https://www.med.unc.edu/it/guide/operating-systems/how-do-i-find-the-host-name-ip-address-or-physical-address-of-my-machine/#:~:text=Windows%2010%20Instructions:,and%20the%20switch%20of%20%2Fall. 

This may not be the answer you're hoping for but Device Preview is not a reliable test. 

For best results, test performance in real browsers and actual mobile devices. 

I use a local testing server.