Yes I also meet the same problem, should ask TP-Link for solution.

Do you mean the threshold settings? From the official website sound detection threshold has been removed. Modifications and Bug Fix: 1. Use H5 for live view on Web-UI. 2. Replace threshold settings...

NC250 supports only 2.4G. Did you use a 2.4G network? If so, try to change the operating channel and retry.

Sounds strange. With" remote control" is switched on, if still can't access the device outside, better contact support team, maybe they can find out what happened to you.

Update the kasa app to the latest version and see if this problem has been fixed. (v1.10.4 for android )

When Kasa showed the device offline, did your phone still in the same network with your plug? After configuration, you should enable romote control first.