you could try to uninstall the utility to see if it will work stably. You can also try this way to connect the wireless network:...

You can try to change the Connection Mode to Connect Manually

it is an unmanaged switch. i think there is no way to change the settings. internet cable -> switch -> PC: the internet speed is (1.5/3)mbps? how did you test the speed?

TP-WDR3600 does not have V4. there is only v1. so i think you did not upgrade incorrectly. any problem occurs after you upgrade?

When you change it the WAN only, the ethernet port on the router will be WAN port, and you can only access internet or the interface page 192.168.0.254 by the wireless connection. the ethernet port...

If you unplug the third one from the socket, how about the speed? You can use the utility to reset the third adapter.