I am assuming that the u verse modem connects to your isp and your router or routers connect to the u verse modem. The U verse modem must bind to your isp and this is usually plug and play. If not call your isp and they will help you thru this process. The other option is to look up the mac address on the old u verse modem and substitute that mac to your new u verse modem in a process called "spoofing". Once the new u verse modem is bound then hook up your routers and you are good to go.
You want a modem that is supported by your ISP. Office Depot may or may not have a modem that is supported. Go to your ISP web page and find their supported modems. Once you are up and running a spare modem does not help you if your ISP goes down. It is still a very big single point of failure. I use a Verizon Hot Spot as a backup to Time Warner. I wireless it to a bridge/router so that i have hardwired Ethernet ports. With this configuration i can run my internet phone, OIS and all my network requirements in case Time Warner takes a dump.
I had the u400 bundle and had all my own equipment and it worked better than atts. And if it broke take to best buy and warrunty it and i dont have to wait 2-3 days for att to show up to replace with their crap. I now run comcast with my own equipment no problems