The fact that the Extreme works points to the problem.. are you powering down the cable modem for that to happen or just plugging it in??
If you just plug it in.. and away it goes, that means you either have not powered off the modem.. ie it has battery backup so powered down does not actually turn it off. Or the ISP does not reset the connection with just a 15min power down... some take much longer.. try overnight. Then ring the ISP and ask them to reset the connection.
If you are in fact powering down the cable modem to get the Airport Extreme to work. but it is failing with the TC when you try it.. then I suggest something is statically set in the computers... make sure all are set to pick up IP automatically.
Green on the TC is pretty much only possible if everything is working.
But do a comparison.. look particularly in the airport utility summary page for the IP of wan port when the TC is connected and the Airport is connected.. this will tell you what is happening. If in both cases you have a public ip then it should work fine.
Also is the cable modem also a router?? What is the model?
The Extreme works only if I power down the cable modem and open AirPort and reconfigure--it does not work straightaway after TC is removed.
The cable/internet modem is model: RCA DCMA25-not a router. I have had it powered down as long as two hours with the same results. There is no inner battery; when it is unplugged it is dead.
With both the Extreme and TC, the AirPort pick up IP automatically. I see my network and have a strong signal in both cases as well--just no internet with TC. I assume I have a public IP 69.xxx.xx.xxx but dont' know why things are not working. Green on the TC should mean everything is working.
I will leave the internet part of the modem off until Friday--I will not be available to deal with the problem until later in the week. I will call Comcast and ask them to reset the connection and report back. Any other suggestions would be appreciated. Thanks in advance.
The problem should not be with comcast. I've configured for them hundreds of times.
BTW, are you sure that 69.xx.xx.xxx was not a 169?
Anyhow, what I suggest is do a FACTORY RESET on the TC. Then re-do the set-up. You also may be doing things quite wrong, so I suggest use an iOS device so there are less options to faul that up.
Set up your Wi-Fi network with your iPad, iPhone, iPod touch, or Mac.
I agree with you, I do not believe the problem is with Comcast. My Wi-Fi network has been working fine until trying to replace my Airport Extreme with Time Capsule.
I have done several "FACTORY RESETS" on the TC as you suggest and then re-done the set-up. I open AirPort Utility and allow it to configure the TC automatically not manually and get a green light but still no internet connection. I have followed the intructions to the letter. I'm not sure what you mean by "doing things quite wrong"--I'm not setting up a new Wi-Fi network from scratch, I am simply adding a TC to an existing one. When I reconnect my old Airport Extreme the internet works just fine. Frustation is starting to set in big time!
What extreme are you using?? Is it the old G model? Or early 10/100 N model.. ?
I suppose the one thing I would try is setting the WAN port speed, try 100mbit and 10mbit and the full and half duplex settings..
Otherwise bridge the TC and plug it into the extreme.. gives you a working router and TC.. maybe not intention but how it works.
Small chance that there is a defect with the TC, but most times it's user error here somewhere. Since it's a new TC, you get at least 3 free months of phone support from DoP. Give them a call, you'll be transferred to wireless department, and they can step by step walk you through it on the phone to make sure you're doing it right.