1. There is a Logout button, isn't it? Logout first and you should be able to enter again.
2. When you cannot serf the internet, can you login its web interface and check if there is WAN IP?
Firmware of your WR740 is up to date?
http://www.tp-link.com/en/download/TL-WR740N_V5.html#Firmware

Thank you for your answer.
1. I can only enter the STATUS page. And when I click on Logout - It writes me again: " You have no authority to access this router!"
2. you were right! the firmware is not updated. I downloaded the file from the link you gave. but it is a bin file and i cannot open it!
so still with the same problem.
I'll appreciate your and everyone's help.
thank you.

Hi Vivian
1. The Logout button sends me to the page where it is written:

"You have no authority to access this router!"

2. I updated successfully the Firmware version - but it didn't help.
The only page Im allowed to see is the Status page.

After firmware upgrade, did you reset the router to factory defaults first?
Your topology is as below, right? MAC is the only client in local network? Can you try to login the web interface on different clients?
DSL Modem(LAN)----(WAN)WR740N(LAN)----(LAN)MAC

Hi.
I guess the update did worked eventually :)
I don't know why... but it took some time...
So for now - so far so good. hope it'll continue that way.
Thanks for your help.