I don't own a 3808, so I can't offer any specific advice. But I can ask some general network troubleshooting questions that might help...
Is this a combination DSL Modem/Router from AT&T or do you have a separate DSL modem and router? What are the brands and model numbers of the gear you have?
You've first got to make sure that the 3808 can 'see' your router.
Look at the status lights on your router. Specifically, the port that the 3808 is plugged into. Make sure the port's status lights are lit up. If not, then your 3808 isn't even physically connected to the router. Check the cables and connections.
If that port is lit up...
Are you using DHCP on your home network? If so, the 3808 should be set appropriately. I have no idea how that's set up on the 3808, but there ought to be some sort of 'automatic' network setting. There should be a way for you to access the DHCP client list on your router. Verify that you can see the 3808 on that list. If you can't see it, then it's not connecting to your router properly.
If you're not using DHCP, and using static IP addresses, then you need to be very sure that you're assigning the 3808 a free IP address and that the subnet, DNS, and gateway settings are correct on the 3808.
Once you can prove that the 3808 is 'talking' to the router, then the next step is internet connectivity. Does the 3808 use a specific port? If you know, make sure the router's firewall is setup to allow it. UPnP is a big help here. If your router doesn't support that, I'd get a new router. See if your router has a DMZ feature. By putting your 3808's IP address here, it will bypass any protective firewalls. See if that works. If it does, then you've got a firewall problem. If not, then you've got some sort of general connectivity issue, and that's going to be really tricky to help with.
Check your router's firmware and make sure that it's completely up-to-date. That matters. I once bought a new notebook that just wouldn't connect to my router. After hours of troubleshooting, I discovered that it was a known issue between the router and the NIC card in the notebook. A firmware update for the router was available, and it completely solved the problem.
And lastly, it's possible that for whatever reason your router & 3808 just aren't going to get along. In that case you're going to need a new router. Do you have another router you could try?
