Let's get to the next step and that is to configure the TC8350C and the TC into a roaming type network. Start by keeping both the TC & Express powered-off. (Note: We will get to the Express as the last "step" and concentrate on just the other two devices.)
- Perform a "factory default" reset on the TC. After it has completed, power it down.
- Now connect an Ethernet cable between one of the Ethernet ports on the TC8350C to the WAN (circle of dots) port on the TC.
- Power-up the TC. Give it about 5 mins to initialize.
- Run the AirPort Utility on your Mac or iPhone.
- Select the TC, and then, select Edit.
- Go to the Network tab. Change Router Mode to: Off (Bridge Mode)
- Go to the Wireless tab. Verify or change the value for Network Mode to: Create a wireless network.
- Enter the wireless network name you noted above in the Wireless Network Name field.
- In the Wireless Security field, select "WPA/WPA2 Personal."
- In both the Wireless Password & Verify Password fields, enter the wireless security password from above.
- Click on Update, and allow the TC to restart.
Just a note here to mention that when the TC has been reset back to factory default settings.......it will not appear in AirPort Utility, so it is not possible to Select, Edit, etc and perform that steps that are listed above at this time.
Instead, the setup "wizard" must be run to provide basic settings to the TC.
The setup wizard can be initiated by opening AirPort Utility and then clicking on the Other WiFi Devices button, then clicking on Time Capsule.

That will get the setup wizard up and running. It will suggest the correct setup in a window that looks something like the screenshot below, except that you will see the Time Capsule pictured, instead of the AirPort Express in the illustration.

For Network Name........Type in the exact same name as the Comcast wireless
For Base Station......Type in the same that you used for the Time Capsule before
For Password........Type in the exact same password that the Comcast wireless is using
For Verify.......Type in the same password again to confirm
Click the Next button in the lower right corner
The setup "wizard" will automatically configure the TC in Bridge Mode, so you do not have to worry about this. The "wizard" will use the WPA2 Personal wireless security setting, so if WPA/WPA2 Personal is required to match the Comcast modem/router......then you can go back in using AirPort Utility and adjust the security setting that way if needed.