It's going a bit difficult to guide you about the data table creation but I'll try my best.
Create a new data table for the index pattern which contains the dataset.
On the "metrics" panel: Choose as aggregation: "Unique count" and select device as field.
Go down for buckets and click on "split rows", select "terms" aggregations and choose "site" as field.
Below that push in "add buckets" button, and once again on "split rows", now as aggregation select "Filters", and in filter 1 input the following: "online:true" , then push in "add filter" and type on filter 2 "offline:false" (if you want you can put a label).
Click on the play button and check you visualization
NOTE: Remember that in order to perform terms aggregation you need have "site" field mapping as keyword.
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