Is your only goal to use this in a Dashboard, as the title of your topic suggests? If so, you don't need a variable at all, much less a connector. Just use the "Attribute" tile template on Dashboard for the device.
If you do need a variable for some other reason, you can set its value via a number of means. However, using Rule Machine to set it to a device attribute is probably easiest (this is an option under Set Variable). You can then use the variable directly on Dashboard by using a tile of type Variable rather than Device. (This is why you'd no longer need a connector if you have a variable.)