For your case, you need to use static routing between the two routers. For everything to work correctly, you need to assign RUT950 a static address from the 192.168.3.x range.
You will also need to configure static routing on your home router.
Also, if you want to see devices connected to the rut950 from a Wi-Fi network, you need to register a static route on your home router. You can see how to do this in the documentation for your home router. If you have not changed the standard network to rut950: Specify 192.168.1.0/24 as the destination and Enter 192.168.3.* (your static wan address of rut950) in the ‘via’ field.
Best Regards,