Adding a second router to improve performance

A router is a combination wi fi unit and firewall.  If you have poor wireless coverage, the first thing you should do is make sure your existing router is set to automatically select the frequencies used.  For unknown reasons, some installers code the same channels on every router and it causes interference.  Your devices should automatically connect even if you switch it from fixed frequency to automatic as long as you don't change the network name (SSID) or code for it.  Adding another "router" is not the best solution for coverage issues but more needs to be known about your existing setup if this doesn't solve the problem.