How can I access Comcast router settings while it's in passthrough mode?

Hi,

I have a static IP address from Comcast and I've set the leased Comcast router (CGA4131COM) to pass-through mode by disabling the DHCP server and setting a custom firewall setting (no firewall). I've set up my own multi-WAN router / firewall to sit between our local network and the internet. Everything works. Hooray. However, I'm not able to log into the leased Comcast router using my user name and password anymore. 

I can access the login screen by typing the Gateway IP address assigned by Comcast (from our local network only), but when I enter the user name and password that had worked fine previously (back when I was logging into the router with its former local IP address before setting it to passthrough), I get a message saying that the password is incorrect. 

As long as everything keeps working, I don't really need to access the leased router, especially since it's not acting as a firewall or DHCP server. However, I'd likely to be prepared in case something goes wrong and want to take a look under the hood.

How can I log in without resetting the device and recreating the configuration? Can settings be accessed through the Comcast site? 

Thanks for your help!

Paul