The router doesn't have AP mode, disable the DHCP server is another way to make it work as an access point, so WAN port cannot be used when you want to use it as an access point.

If you cannot remotely control them on Kasa, then check if there is any available firmware update. It doesn't seem to be related to that though. Any error message on the Google Assistant app? I think...

What's the hardware and firmware version of your NC250? Could you share a screenshot of the admin interface? Maybe yours looks different than mine.

If the LED is lit, indicating the extender is connected to router, check the IP address of your device connected to RE. If IP address is given by the RE, you may contact tplink support, send them the...

If you connect a pc to the tplink router via cable, would you get over 80? If only WiFi is slow, what's the WiFi signal strength? Make sure you get full bars when you test speed, and don't run any...

any power outage during the firmware update ? Looks like need replace one if keeps on red light