Port Sharing for T-Mobile CellSpot

Since your Comcast gateway (combo modem & router) is in bridge mode, your Apple Time Capsule (TC) will be your "main" router in your network. For port mapping (aka, port forwarding), these then will need to be configured on the TC to allow traffic thru those specific ports to the required device on the local network behind the TC.

The TC's DHCP setting would be configured for your Comcast Internet service requirements. Typically, in the AirPort Utility, you would have Internet > Connect Using = DHCP. Your ISP would then provide the correct WAN-side IP addresses to your TC for it to access the Internet.

For the LAN-side of the TC, you would select Network > Router Mode = DHCP and NAT. Finally, you would enter a port mapping setting for each WAN-side port you need to open on the TC's NAT firewall. In this case, ports 123, 500, & 4500.

Using port 123, as an example:

1. You would first select the plus button under Port Settings.
2. You then would enter a description that makes sense to you for what this port's traffic that will pass thru it.
3. Since you didn't mention if the traffic thru these ports will be either, or both UDP & TCP, enter 123 in the next two windows: Public UDP Ports, & Public TCP Ports.
4. For Priviate IP Address, you would enter the IP address of the device on your local network that this traffic, coming from the Internet, will go to. Note: You typically want this device to have a dedicated private IP address.
5. For the next two windows, you can either enter 123 for both, or whatever port you want configure on the device that will be "listening" for this traffic.