I purchased an arris surfboard modem. I read somewhere that I can connect both the surfboard and the Comcast Arris TG1682G, and set up the OpenDNS on the surfboard admin page and still use the xfinity admins page features. But I can’t find the forum on that again :/
You need a router, not a modem. Once you get a router, connect it to your Comcast gateway and set the gateway to Bridge Mode. Instructions here.
Or even better, since you already bought a modem, return your Comcast gateway, buy a router, and activate the new modem and set it up with the new router. You can save yourself the monthly $11 rental fee by returning the leased gateway.
Would I be able to set up the surfboard as the modem, the Comcast modem/router as the router and my netgear as a range expander? We have a terrible connection throughout the whole house.
No, the Comcast gateway doesn't have a WAN Ethernet port for you to connect an external modem to. You only have room for one cable company interface in the system here - either use the Surfboard modem, or the Comcast gateway, but you can't use both. Since your original purpose here was to install OpenDNS, you still need a router to get that done. I don't know what model Netgear you have, but that might work just for that purpose.
As far as improving your wifi coverage, there's a great instructional here that you can refer to for ideas.
I’m so sorry, just to clarify, so I cannot connect the surfboard to the internet cable for my modem, and connect the Ethernet from the surfboard to the Comcast Arris as the router?
AFAIK the router component / portion of the Comcast combo gateway devices can not be separated out from the modem portion and used independently.