According to the guide you linked, you set up your OpenWrt router with masquerade. In other words, for every connection the RaspPi initiates, the OpenWrt will pretend that itself is making the requests instead of the RaspPi. By design, this doesn't work the other way round: When you access the OpenWrt from your main PC, it's really the OpenWrt; and the IP address of the RaspPi is not accessible.
In general, what you are trying to do (bridging a LAN to a WLAN station in client mode) is difficult to do for technical reasons (the WLAN protocol only uses 3 MAC addresses instead of the 4 MAC addresses needed to make this work properly).
There are workarounds, but they all have their gotchas and drawbacks. Which of them to use depend on what exactly are you trying to achieve by putting a RaspPi behind a WLAN router: Is the RaspPi the only device for which you want to do that? Do you only need to access special ports on the RaspPi? Etc.
Edit
Two options for the purpose mentioned in the comments:
1) To use the old Router as a glorified Wifi-Dongle for the RaspPi, have a look at wlan_kabel. This is a program that uses packet sockets to directly bridge the WLAN network interface to the LAN network interface. As a consequence, the Router won't be reachable over WLAN anymore.
I've no idea if this program is available for OpenWrt, or if you need to cross-compile it.
2) To access the application on the RaspPi that controls the printer, find out which port it uses, then use port forwarding to make this port (or several ports) accessible under the IP of the Router.
