How should my time capsule work with my ISP modem?

It sounds like your Comcast "modem" is really a modem/router, which is also known as a gateway device that combines the functions of a modem and wireless router in one single package. Since the Time Capsule is also a wireless router, you don't really need a modem/router, which complicates things on your network.

A simple "modem" will not produce a wireless signal at all.....it only connects to the Internet. Then, you connect your router to the modem. So, one option that you might have is return the modem/router to Comcast and ask them to provide you with a simple modem.

If you decide to keep the current modem/router from Comcast, you will need to decide whether you want to use the modem/router provided by Comcast for your wireless, or the Time Capsule. In most cases, the Time Capsule wireless will far outperform the wireless on a modem/router supplied by an Internet Service Provider (ISP).

If you find that to be the case, then you can simply ignore the wireless signal from the modem/router, or you also have the option to turn off the wireless function of the Comcast modem/router with assistance from Comcast.

In the unlikely event that you find that the Comcast wireless performs better than the Time Capsule wireless, you can use the wireless on the Time Capsule for a "guest" network, a "kids" network, or other purpose......or......with a few clicks, the wireless function of the Time Capsule can be turned off and it will function as a back up hard drive only.

Not sure why you marked your "thanks" as the solution your question, but glad that the information seemed to help you.

Unfortunately, the bottom line here......although it is not specifically spelled out in the information about your network......is that it seems that things were working fine before the modem was changed, and they have not worked correctly since the new modem was installed.

Yet, Comcast.....if you call them.....is likely saying that everything is looking fine to them......and they are trying to point out that it is the Time Capsule that is the problem.

Is that a fair assessment of about where things stand at the present time?

And unfortunately, they are dead wrong. since they do not even understand what it is that a Time Capsule does.

You have two options:

Option 1)......Return the modem/router....which you do not need...... to Comcast and ask them to provide you with a simple modem (like you had before). A simple modem is just that.....simple.....it only establishes an Internet connection. It does not provide any router functions or wireless services.....and will look something like this, for example:

What you have now....and you don't need, because it makes everything much more complicated.....is a modem/router, which is also called a gateway. This type of device looks something like this:

Option 2)......If you don't want to return the modem/router that Comcast has furnished to you......then you will have to start all over again and reset the Time Capsule back to new out of the box factory default settings......and then set it up again to work with the modem/router that you have.

If you elect this option, my recommendation would be to not turn off the wireless on the Time Capsule, and use the Time Capsule wireless for your network. You can ignore the wireless from the modem/router or have Comcast turn it off.