Port forwarding from one router to router, then to a machine on the second router

The first router is being used as a router, and the second router is being used as a Wireless Access Point (AP) - it doesn't need to be a router, as it resides on the same network as the first router.

Connect the AP router to the first router via LAN ports. Turn off DHCP on the AP router.

Now all the LAN ports both routers, and the wifi network on the AP router are all on the same subnet because they are bridged. They can all use DHCP from the first router. Because in this configuration, you are connecting them as a layer two network, which means that broadcast traffic, such as DHCP requests, can be seen by every device.

Effectively, you can ignore the port forwarding on the second router, and port forward on the first router directly to devices connected to the second routers wireless network.