What you are wanting to do is called turning your "slave" router into an access point. An access point is a wired/wireless extension of your network. With your secondary "slave" router configured as an access point then the primary router will provide all dhcp addresses, qos, port forwarding and all other features for all devices connected to both routers.
To turn a router into an access point you need to configure a few things on it:
1) Set router IP to be in same network as primary router. Lets say primary router 192.168.10.1 with dhcp from 192.168.10.100-.254. so you would set your access point to have an Ip of 192.168.10.2, subnet of 255.255.255.0, and gateway of 192.168.10.1.
2) dissable dhcp
3) dissable NAT and SPI firwalls (not all routers have SPI).
4) plug cable from primary router into a LAN port of the access point intsead of wan
If "slave" router had any QoS, port forwarding, etc configured you will also want to turn that off
You can most likely find a more detailed write-up for your particualr router that takes inot account the settings that can be changed at the same time before needing a reboot as each router model/oem is different with this.