What is the cause of a "No network connection" banner? This banner randomly appears, persists across mobile and WiFi networks, and then randomly disappears, and I don't know where to begin to search for the issue.
In my case the bar is gray and at the bottom. The bar is drawn over all other apps and is always visible.

To check which apps have this capability:


Open the Settings.
Navigate to Apps (under the Personal section).
Open the configuration by tapping on the cogwheel icon in the top right.
Open Special access (under the Advanced section).
Open Draw over other apps.


This already made it much more apparent which apps are candidates for this issue. The next time the issue appeared, I closed apps one by one until the bar disappeared.

In my case, the app causing the bar was Office 365 Admin.