That 18.104.22.168 IP address seems to belong to APNIC - http://www.apnic.net/ - the "Regional Internet Registry that allocates IP and AS numbers in the Asia Pacific region."
The "whois" information in the APNIC web site for that IP address says that it belongs to the "APNIC Debogon project":
I'm posting those "whois" results at the bottom of this post.
What is the Debogon project? The Wikipedia article about "Bogon Filtering" - http://en.wikipedia.org/wiki/Bogon_filtering - says that "a bogon IP address is a bogus IP address, and an informal name for an IP packet on the public Internet that claims to be from an area of the IP address space reserved, but not yet allocated or delegated by the Internet Assigned Numbers Authority (IANA) or a delegated Regional Internet Registry (RIR). The areas of unallocated address space are called the bogon space."
Having said that, I don't know why your TP-LINK router is sending all DNS requests to that IP address. I also have one TP-LINK router and I don't think it's displaying that behavior. When I get to that router, I'll check it.
EDIT: The TP-Link that I mentioned is a wireless router "TL-WR340G" and it's connected to a cable modem. In the router web interface, in the "Network -> WAN" page, I have the "WAN Connection Type" set to "Static IP" and it is set to an IP address of the same network of the Cable Modem. The "Default Gateway" and "Primary DNS" are both set to the IP address of the Cablem Modem. I hope this helps! :-/
APNIC found the following authoritative answer from: whois.apnic.net
% [whois.apnic.net node-5]
% Whois data copyright terms http://www.apnic.net/db/dbcopyright.html
inetnum: 22.214.171.124 - 126.96.36.199
descr: APNIC Debogon Project
descr: APNIC Pty Ltd
status: ASSIGNED PORTABLE
changed: firstname.lastname@example.org 20110922
role: APNIC RESEARCH
address: PO Box 3646
address: South Brisbane, QLD 4101
remarks: + Address blocks listed with this contact
remarks: + are withheld from general use and are
remarks: + only routed briefly for passive testing.
remarks: + If you are receiving unwanted traffic
remarks: + it is almost certainly spoofed source
remarks: + or hijacked address usage.
remarks: + http://en.wikipedia.org/wiki/IP_address_spoofing
remarks: + http://en.wikipedia.org/wiki/Regional_internet_registry
changed: email@example.com 20110822