I was getting a lot of the offline red exclamation points for a while recently. What I have done to completely correct them is to install Pollster and add the devices as they go offline. I have about 6 devices that were going offline consistently and after setting pollster to poll and refresh them they have not dropped offline once.
It is kind of frustrating that we have to use third party tools for basic functions like this, but at least we have the ability to use them. Unlike many other locked down smart home hubs.
Here is the link to pollster for anyone that needs it.