That setting you have listed is just there to ensure that you can access the setup screen(s) on your modem itself.
For instance, I also have an RT N16 and Tomato. My cable modem is in bridged mode. |I has an internal IP address of 192.168.100.1; my rotuer is 192.168.2.1 and all devices onmy LAN are 192.168.2.2 - 192.168.2.254.
That checkbox just ensures that from my devices on the LAN I can access the modem config on 192.168.100.1
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