How do I extend a network with an AirPort Extreme from an Arris TG862?

I'm using Comcast as my ISP, and after some issues with a previous Motorola modem, they supplied me with an Arris TG862 Router/Modem combo tower. I have two AirPort Extremes, one of which was previously connected to the old Motorola modem to send wireless into the house, but it only reached halfway, so I had the other AirPort Extreme in the middle of the house to "catch" the signal and then broadcast it out to the rest of the house. This worked fine for a while.

But with the new TG862, it doesn't seem to be an option. From what I've read, if I have the TG862 in one room (connected to the cable), AirPort Extreme #1 in the middle of the house, trying to join the existing network (or extend it?), and AirPort Extreme #2 in the far end of the house (to extend from AirPort #1), it won't work. Apple's "extension" technology isn't compatible with all other modems or routers, and even if it was, I'd have to somehow configure the TG862 to "allow to be extended."

So my question is, IF there is a desktop utilizing the TG862 as a direct connection (because it has no built-in Wi-Fi), is it possible to put the TG862 into bridge mode, connect one of the AirPort Extremes and have it act as the router so I can use AirPort Extreme #2 to extend that network out to the rest of the house? I want to ensure the desktop still has a connection, but there's Wi-Fi that actually goes out through the whole house.

The connected-to-the-cable-desktop is a PC; we have multiple iPhones, iPads, and anywhere from 1 Mac laptop to 1 iMac in the house at any given time.