Have you tried to reset it to factory default?You can press the reset button for about 15s.Then try to login the interface again.

I must bump this thread because I have a new, identical TL-MR3040 V2 with a login problem also. When I first tried my 3G modem with the router (with very old, possibly original firmware) and could not get it to work, I immediately updated the firmware to latest mr3040v2_en_3_17_1_up_boot(150921).bin as cmahendra. I then got everything to work after EXCEPT I can't seem to change the router's login credentials away from the default!

If I try to set the User Name and/or Password to anything else at all I find I CAN NO LONGER LOG-IN TO MY ROUTER i.e. it refuses to accept the different creds I entered, nor of course do admin/admin work any more. So I have to do a Factory Reset to regain access to the router (with of course the defaults again).

Does anyone have any ideas how I might change my router's credentials so it does no longer have to use admin/admin????