This will probably work with other hardware too, I don't know.
1. Reset your ddwrt router to factory (I don't know what you might have changed that will affect these instructions)
2. Do a factory reset on your cable modem (this seemed to be important)
3. Log into your CG3100 - default username and password is: admin and password
4. The last setting on the side menu is "NAT", uncheck this - your "router/modem" will become a simple dumb modem.
5. After 5 minutes power cycle it - why not.
5. Check this works by plugging a laptop into one of the network ports on the back and check you can access an internet site, choose a simple one like mine, BE CAREFUL - if your operating system asks if you trust this network say NO - your computer is connected to bare internet with no firewall.
6. Unplug from the net if all is good. If not repeat the steps 2-5 until good.
7. Plug your lappy into the ddwrt router, check the light on the front shows this has worked.
8. Log into ddwrt - default username and password is root and admin - default IP is 192.168.1.1
9. Change the WAN connection to Automatic Configuration DHCP
10. Give your router a name and a hostname
11. Set the local address you want, or leave it as is if there is nothing else on your LAN expecting anything here.
12. Leave 0.0.0.0 as it's own gateway.
13. Set your static DNS to OPENDNS, stop your ISP snooping on your browsing- 220.127.116.11 and 18.104.22.168 - http://www.opendns.com/opendns-ip-addresses/
14. Set your NTP server - I used to like the CSIRO's National Measurement Laboratory - until some jerky company decided to make them the default in their devices and then sold thousands of them, flooding their bandwidth, so just use 0.au.pool.ntp.org
15. Set your timezone and DST times, in AU daylight savings is the first Sunday in October to the first Sunday in April. Eastern Standard Time is UTC+10.00
16. Click SAVE
17. go to ADVANCED ROUTING and change operating mode to GATEWAY
18. Click APPLY and the unit will reboot. At this stage you must remember the IP address you gave your router (if you changed it)
19. Log back in, go to WIRELESS and change it to Disabled (or set your security - that's up to you but I prefer to have a separate AP) click APPLY
20. Go to ADMINISTRATION and MANAGEMENT and set a unique username and password - not your firstname and monkey123, but something hard to guess hey! Click apply.
21. That should be working now, if you plug your router's INTERNET connection into one of the ports in the modem it (fingers crossed) will be online.
22. Check this by going to ADMINISTRATION and COMMANDS, enter ping 22.214.171.124 and hit RUN COMMANDS - if this works your router is online.
23. enter ping google.com if this works your DNS is working too.
24. Plug a laptop into one of the other plugs in the router, bring up a shell (windows key, then type cmd) and enter in the same two commands.
25. If this has all worked then you should be online, bring up a webpage on your lappy and pour yourself a glass of milk!