How about status of wireless network connection on your laptop? Can you connect the laptop to LAN port of WR740N and check if you access internet? I doubt it is a computer issue...

Yours is V2 but the 450Mbps one is V3. V2 can only upgrade firmwares for V2. http://www.tp-link.com/en/download/TL-WR940N_V2.html#Firmware

None of them can be displayed... Have you contacted support.fr@tp-link.com?

What's the operating system of your PC? Did you install the utility? It seems that Security Type and Encyption Type can be modified... http://www.tp-link.com/en/faq-767.html

How many wireless clients are connected at the same time? Have you tried to reduce the number of connected clients to see if the connection will be more stable? Try to change channel to 1, 6 or 11....

Wired connection and 5GHz Wi-Fi are working OK? Have you tested 2.4GHz Wi-Fi by any other wireless clients? Try to change channel width to 20MHz to see if it will fix the problem.