The problem you experienced is a known issue when setting up your own router behind the ATT Uverse router. Ran into the same problem when I initially setup a TomatoUSB firmware router behind an ATT Uverse 2Wire 3600HGV gateway/router. There are several earlier threads both here and via a Google search the explain how to setup a 3rd party router (ie your own) behind an ATT Uverse router and to use the iptables setting you posted to deal with the connection drop issue every 10 minutes problem. The cause as explained in one of the links below is due to; "the DD-WRT SPI firewall is blocking responses from the DCHP server. UDP requests are sent to the DHCP server at 50% intervals, but the response is actually coming from a different server, and is thus blocked by the firewall."
Someone please sticky thisHelp for u-verse noobA Google search for DDWRT and ATT Uverse would yield these links:
AT&T U-verse with DD-WRT router in DMZ modeAT&T U-verse 2wire Gateway and DD-WRT Router in DMZ Mode
AT&T U-verse 2wire Router and WRT54GL with DD-WRT ProblemsIf your using a 2Wire 3600HGV, there is a huge thread over on the ATT forums site that discusses how to setup a router behind the 3600HGV.
U-verse for BUSINESS? : 2Wire 3600HGV bridge mode? or another AT&T supported VDSL modem?Edit to add: The 10 minute drop issue is also mentioned in the forum AT&T U-Verse FAQ:
How do I configure bridge mode (DMZ+) on a 2Wire 3600HGV, 3800HGV, or 3801HGV?"8. Restart your router, when it gets an address via DHCP again, it will be the public outside IP address. At this point, you can leave your router in DHCP mode (make sure the firewall on your router allows the DHCP renewal packets, which will occur every 10 minutes), or you can change your router's IP address assignment on the WAN interface to static, and use the same settings it received via DHCP."