There are at least two separate things here: they show offline in the IDE and they are not visible in the app. Being offline should not make then disappear from the app. Are you sure you are looking in the right place? The device for the Z-Wave repeater side does nothing (e.g., doesn’t control the device), so I don’t put it in a real room but rather keep them grouped in an “unused” room. Are you sure you didn’t put them somewhere? (The Zigbee side really controls the outlet, so that’s the useful device to place in a room).
Just as a general rule, anything multiple things break in SmartThings I avoid trying to make local changes, because you never know what has been temporarily broken in their infrastructure. It saves a lots of frustration. I’d avoid excluding them, re-adding them, and instead wait a few days.
The repeater function on these plugs is totally independent from the DTH, so I strongly suspect that it showing “offline” does not necessarily mean there is anything wrong with their operation.
As for them going offline: What does the ‘events’ tab on the device say in the IDE? That can be a hint.
The one thing I did that fixed mine from being marked offline was to switch to @SteveWhite’s DTH with diagnostics.