Yes, with newest firmware, you can switch off 2.4G or 5G.

There is no ACL on Deco. You can block devices. Tap on the unknown device, then tap the three dots on the top right, you will see Block. Deco has antivirus powered by TrendMicro. It's not that easy...

可以有線連接，6顆在一起組網，在一個mesh裡面，是共用一個SSID的。

May test another one. A comparison test is a good idea.

A7 C7 are different models. And connection dropping is a bit different from the issue that speed drops dramatically. If you have connection issue, check it out....

Only way is to hard reset the extender.