I would first try turning off all smart plugs then removing them/forget device from the device list in the xfinity app. Then factory reset all smart plugs one by one being sure to name each one as its added to your network in the xfi app. Merge the 2.5 and 5 ghz network as this is definitely not recommended and will cause a lot of problems with devices trying to figure out which band to connect to. Smart band was created for good reason. I understand this is needed to install some smart devices however most offer an API hotspot setup that will work on merged networks if it's not finding the 2.4 band. Then test the device in the app meant for that plug to make sure it's communicating. If it is ask alexa to discover devices after which they should show in the alexa app. I would start there and see if it fixes the issue.
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