Did you place a line like this in your config.ru?
set :outboard_file, 'http://example.com/path/to/your/outboard.txt'
It sounds like you did, but I’m just double checking.
If that setting is pointing to an accessible file in the expected format, then I’m not sure what the problem is. I tried looking into it, but my test version of Dashing isn’t working at all. Unfortunately, the Dashing project hasn’t been supported since 2016, so I suspect you might be right about the newer versions of ruby causing some issues. I suggest looking for a dashboard that is still being actively supported/developed. Sorry I don’t have better news for you…