Are you changing the DNS setting on the DHCP screen or someplace else. The default setting on the dhcp screen should be blank not 0.0.0.0....I think its been a while since I used tplink.
If all else fails I would use the reset button on the back and factory reset it. That should fix the password problem also since it will be set back to the factory default. I would also check for a firmware upgrade since maybe it is a bug.