Ben,
Above you stated that you have deployed Military Tools 2.0 Widgets to your 10.5.1 Portal and you are not having luck with the Visibility widget. Have you tried the Visibility widget inside of Web AppBuilder for ArcGIS (Developer Edition)? If this is not how you deployed the widget could you please share your workflow so I can better trouble shoot the problem?
Also, if you did not use Web AppBuilder for ArcGIS (Developer Edition) could you attempt that as well to see if you get the same results? Here is a link that will walk you thru the steps:
Configure Web AppBuilder for ArcGIS Developer Edition
The guide uses an online organization; however, you should be able to use the same steps with your portal.
Hope this helps find the root of the problem
Thanks
Lyle Wright
Defense Solutions