iOS Orbi app not showing attached devices (RBK853)

My similar problems were resolved with the recent firmware updates (the one you're on now, and the one before it), however there may still be some related issue.

You say the web interface shows the devices; if you click on each one, can you check if any of the fields contain any non-alphanumeric characters?

In my case the ampersand '&' character in a device manufacturer name was breaking the XML format that the app reads/writes to (I had to escape or replace it and save in the web app, and then everything showed up in the devices list on the app).

There can be other parts of XML entity names that might be breaking it that weren't fixed, like greater/less than characters, single/double quote characters, or more if they happen to be in the wrong place of the data the app receives.

So a quick check through each connected device on the web app and replacing/saving any non-alphanumeric character instances you find could help confirm or rule out if that's still an issue in some cases.

Beyond that, if you're familiar with it (or feeling adventurous enough), running a tool like Fiddler on a machine and setting your IOS device to use it as a proxy, would allow Filddler to show you the data the app is requesting/receiving to spot any oddities or error codes the app isn't displaying.