So I guess the OpenWRT has the Fritzbox as default gateway, right? In that case, you need to just add a static route to your Fritzbox so he knows that there is a 192.168.1.X on the interface that connects to the OpenWRT. But if that traffic is being tunneled the VPN, it won’t work.
You should set up the OpenWRT (as is, the most capable device) at the internet gateway and just use the Fritzbox as a repeater. If you need the Fritzbox as a cable modem then you are out of luck and probably need to buy a new device.
In your place I would ditch the Fritzbox as much as possible (as is, use it just as a modem) and connect everything to the OpenWRT. If you have special needs, the OpenWRT can also create more than 1 SSID with different networks. Definitely buy new hardware if needed.