I've been trying to set up Internet in my new house and in my troubleshooting I hooked the modem up directly to the supply line coming out of the ground and it connected successfully. If I hook any coupler or splitter beyond that then attach another cable, it fails to connect every time. Could it have anything to do with the gauge of the cable, or maybe the dB rating on the splitter? The modem is a Motorola SB6121.

Might need a multiport router or a LAN Bridge?

MmmmmisterCrow
I've been trying to set up Internet in my new house and in my troubleshooting I hooked the modem up directly to the supply line coming out of the ground and it connected successfully. If I hook any coupler or splitter beyond that then attach another cable, it fails to connect every time. Could it have anything to do with the gauge of the cable, or maybe the dB rating on the splitter? The modem is a Motorola SB6121.

You will most likely need a powered splitter if you are having issues with connectivity once you split the signal. You can just call your ISP and tell them you want to relocate your cable modem and they should be able to come out, test where you want it to be located, and add the signal amplifier where necessary. They needed to do that in my house considering my original location was downstairs and now my office is located on the second floor. Without an inline amplifier your signal is most likely degrading below thresholds and can't connect because of that reason alone.

MmmmmisterCrow
Turns out the problem is on Comcast's end.  Who would have thought my first experience with Comcast was going to be terrible?
 
A tech is coming Thursday so I'll just be without internet at least a few days.  Until then it's the dark ages for me.

Give them time and they'll lower your expectations of their service.
 
Good luck getting it resolved. I'm sure it'll be just fine.