I have a TP-Link router and the default IP address is 192.168.0.1 or tplinkwifi.net. I have since changed the IP but when set in default mode I had to log in to the router user interface with the address above.
Do you have a TP-Link extender? If you do you need to read the manual for your extender.
From the TP-Link website:
"Access the web interface by using the default IP address 192.168.0.254 or default domain name http://tplinkextender.net (the default IP/domain name is also printed onto the label attached at the bottom of the TP-LINK device)."
You need to be careful as there is a malicious website under the same address. If you are logging into the extender UI you may be ok, if you are going out to the net then you are not.
This Amazon review of an extender may help
http://www.amazon.com/review/R2VUT8M8YHX13B