Welcome to 2.4ghz hell.
99% of automated devices, smart plugs, thermostats etc use legacy 2.4ghz.
Most modern routers and APs use both 2.4 and 5ghz, and do a mediocre job trying to band steer to 5ghz. ISP supplied routers with built in wifi are the worst.
If you're having trouble getting these devices to connect see if your router or AP allows you to set up a custom SSID with just 2 4ghz channel. Also use a 12 character or less password since some Zwave based devices have trouble with longer passwords. Typically this solves the issue 85% of the time. Even my $30 Tplink router can do this.