Did the service work better before the router was installed?
What type of DSL modem do you have?
What is your DSL rate that you are signed up for?
Is your wireless running encrytion? Maybe you have a someone else using your wireless?
You need to perform a bandwidth speed test.
http://bandwidthplace.com/speedtest/
or
http://www.dslreports.com/stest
I would perform the speed test with and without the router in place to see if there is a difference.
Perform a tracert, find out what the first hop is beyond your DSL modem on the Telco side. Ping the Telco first hop and see what your ping times are.
Also see if you can access the DSL modem web page for line stats (noise margins, CRC and FEC errors)
Sounds like you have the usual impaired DSL that works no better than a 56k dialup!
JamesO