Try and disable the Antivirus temporarily, see if that's going to make a difference. If not, report this to TP-Link support, I am sure they can help you on this.

Hi SirSpidey, sorry, but I still don't see what the problem is? As the Deco always broadcasts 2.4G WiFi signal, so both the plug and the phone should be able to connect to the Deco network.

A topology of your network would help understand your situation better, if you dont mind.

Yes, I believe there's a limit, the limit is 16.

1, Manual up/down QoS bandwidth limits, No-ip support, you should feedback to TP-Link support. 2, IP range, I hear they are planning to add this to the Deco. 3, Loopback, Port being forwarded on the...

There is alway two WiFi networks from the Deco, but with the same name. So dont worry about it, you smart plug will be able to see the Deco WiFi signal, no issue at all.