To access the OBi200 you need to do port forwarding in the secondary router. The OBi200 defaults to port 80 for Web access. I assume your secondary router also is using port 80, so you need to use a different port to log into the OBi200. The following example uses port 800.
Main router: 192.168.1.1
Secondary router's address on main router: 192.168.1.50
When the secondary router is connected to the main router, an IP address will be assigned to the router within the main subnet.
Secondary router 192.168.2.1
Port Forward: port 800 to 192.168.2.100 at port 80
Log into OBi200:http://192.168.1.50:800
If your router doesn't have the option to port forward to a different port, you need to change the port on the OBi200.
System Management -> Device Admin -> Port: 800
Port Forward: port 800 to 192.168.2.100