It seems that error should not happen when using WPS. Something is wrong but I don't know what it is. You can contact the tplink tec support directly. Take a picture of the error and send it to them....

If the main Deco is still green, it should work even though the 2nd deco is red. Did you reset the 2nd deco? Push the reset button underneath for 1s to reset it. Wait unitl the light turns blue then...

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.