While the extra info is nice to have there is no router made that can break the rules or limits placed on power.

This means that to solve this we use the usual solution of a second WAP. Move the current ADSL+WiFi router to one area and let the other get served with another WAP.

Only your newer network will search for a better router or hope such will cure coverage issues. This is one of those well done areas but I understand you may be new to WiFi networking. here's a primer for you on the issue.
http://now.avg.com/boost-home-wi-fi-signal-strength-part-1/

Heatmapper can help you put your existing router in a good spot for one area and then the second WAP to cover the other area.
Bob