Having issues connecting SRX300 to Xfinity router in bridge mode

Hi all!

I acquired an SRX300 some time ago from an old friend of mine so I could try and learn it. After some 4 months of procrastination, I have finally gotten around to setting it up and configuring it, but for some reason, I can't seem to get a public IP address out of the Xfinity router through to my SRX.

What I've tried so far is using the default configuration where ge-0/0/0 runs under untrust and is using DHCP. I've also attempted to set it with a static IP address, as when I tried to connect my main PC directly to the router, it required that I manually set my IP address instead of using DHCP before it connected. I've also attempted to disable auto negotiation, but rolled it back after nothing came of it. To the best of my knowledge, I'm connecting to the Xfinity router directly as it's acting more as a modem than a router at this point, so I don't think I would need to whitelist the MAC Address with it.

Does anyone have experience with setting it up with this sort of configuration? Will try to update further with proper configurations and whatnot as soon as I can, currently stuck to configuring the firewall through the serial USB connection on the front.