just to make it clear..
ang gusto mo mangyari, yung 2nd router mo will serve as a switch dba?
1. Disable DHCP Server on 2nd router
2. connect a UTP cable(LAN cable) from any port of 1st(main) router to PORT1 of 2nd router instead of connecting it to WAN Port.
in that case ung 2nd router mo sa 1st floor nio magiging switch nalang.. ang DHCP nia ay galing na sa main router mo.. yun na mag assign automatically ng IP addresses ng lahat ng PC mo..
hope this will help.. yung ibang configuration na gusto mo cguro alam mo na un..