There is always a firmware upgrade option on system tools. What do you see on system tools? If your devices cannot connect 5GHz, change 5GHz settings, like SSID, security type and channel. Same for...

NC450 doesn't store video on the cloud server. You can put in a sim card and enable 24-hour recording, photos/video will be stored to sim card. Once you use up 60 email alerts, an email for renewal...

Update your NC250 firmware. web plugin is not required any more.

Try this link http://www.tp-link.com/en/download/TL-WPA8730-KIT.html#Firmware

There is no way to buy it now. You can contact the support in UK and confirm whether it can be bought.

Do you mean connect a switch in PLC? Yes, the PLC can be regard as a cable, if you are sure that it is not forming a loop a loop.