BTW, the router should be doing NAT, so your ISP won't see the local addresses at all.
To change the address range… source AirPort Utility 6.x: Set DHCP options for your base station or your AirPort Time Capsule
If you’ve set up your base station or your AirPort Time Capsule to use Dynamic Host Configuration Protocol (DHCP) to provide IP addresses to computers and devices on your wireless network, you can choose the range of IP addresses a device uses, determine the amount of time a computer will have a valid IP address, enter a welcome message, and reserve specific IP addresses to assign to specific computers that join your network.
- Open AirPort Utility, located in the Utilities folder in the Applications folder. Select the device you want to set up, then click Edit. Enter the password if necessary.
- Click Network, and then choose either DHCP and NAT or DHCP Only from the Router Mode pop-up menu, depending on how you want to use DHCP.
- Choose a range of IP addresses from the DHCP Range pop-up menu. Choose 10.0, 172.16, or 192.168, and then enter a beginning and ending address in the beginning and ending fields, depending on which addresses you want the device to provide.
If you set up a guest network, you can choose the address range for the guest network as well.
- Enter a number in the DHCP Lease field, and then choose minutes, hours, or days from the pop-up menu.
- To provide specific IP addresses to specific computers on your wireless network, click Add (+) below the DHCP Reservations list, and then follow the onscreen instructions to name the reservation. Reserve the address by MAC address or DHCP client ID. If you choose MAC address, click Continue and enter the MAC address and specific IP address.
It would be unwise to have the actual ranges overlapping. Set one to e.g. 18.104.22.168.2-10 & the other to .11-20
& don't have 2 devices both trying to do NAT, leave that to the main router.
Also, make sure the main router is only handing out addresses for wired devices & the TC for wireless, or they'll fight.