Regarding the disconnection of the kasa wifi switch or plugs. I used several kasa wifi plugs and this is my experience:
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Is it shown offline in smartthings and in the kasa app or only in smartthings? In this case, it could be a problem with the services linked to kasa smart app and it could be solved by refreshing the link in settings.
It ever happens that one or more devices show offline only in Smartthings.
Something similar also happened, time ago, with meross linked services, they do not show offline, but it does not update states of some devices. -
Does the switch lose the Wi-Fi connection with the router? In this case, as it has coincided with disconnections of zigbee devices, it could be that your router manages the 2.4 ghz wifi channels automatically and has changed to a channel that may interfere with the hub 2.4ghz zigbee channel and has caused these problems.
About this there is a post by @nathancu , which explains it very well, I really liked it.