The issue comes from USB sharing. I found tp-link router uses "admin" as default USB sharing password and also the web interface login password. You can change it on System Tools-->Administrator(some...

It seems tp-link canceled this function. I noticed the Emulator also doesn't have this feature anymore: https://www.tp-link.com/resources/simulator/C9v1_simulator/index.html But no reason

the other one will not require another router, but are the downstairs and upstairs in the same power circuit? if not, the connection between the adapters might be lowered down or blocked

yes, actually the TL-PA4020P is just like an ethernet cable, you may connect one of them to your router, the other one to the computer, as long as the two units are paired via pair button, internet...

you may also try to turn off firewall or anti-virus, besides, we don't have to use the utility at all, as long as your two units are paired with each other via pair button, the internet will work

what is the exact model of the extender? such as TL-WPA... or TL-PA... the one with W means it has wifi, otherwise, there is no wifi feature at all