Upgrade the latest firmware first. http://www.tp-link.com/en/download/TL-WR940N_V2.html#Firmware Reconfigure the router to see if it will work stably.

Follow this guide to do troubleshooting. http://www.tp-link.com/en/faq-785.html

I don't think so...admin is the only default username and password. Do you remember the version of previous firmware? You didn't change the login username and password before upgrade, right? Can you...

So the laptop deletes network profile automatically after your shut down the laptop? How interesting... Other wireless clients work OK except the Samsung laptop, right? Is it possible for you to...

Archer C7 V1 has no firmware to support Unifi but Archer C7 V2(Archer C7_V2_140318(Unifi & Maxis Fiber)) supports Unifi. http://www.tp-link.com.my/support/download/?model=Archer+C7&version=V2#tbl_j

How about your test in the weekend? :)