Question: I have a modem+wireless router that my ISP gave that I use to connect to the internet as well as connect my IP TV box. 

I just bought a Time Capsule and I want to use it as the wireless access point. Unfortunately I am having difficulty getting the TC to connect to the internet. I have already turned off the wireless on the ISP router but I am afraid if I mess with any DHCP settings I might lose access to my TV set top box; ditto with bridge mode. However I am open to any suggestion.

Isnt there a way for the TC to connect to the internet over my existing router and use a different IP range to allow wireless devices to connect to it?

Answer: 
Well, as it turns out there IS a way to do this.

Connect the TC to the ISP's router using an ethernet cable. This will cause the ISP's router to allocate an IP address for it.
Go into your ISP router's settings and find the place where you can reserve an IP. It will probably ask you for the IP and MAC address. You can find these details on the page where the router shows all connected devices.

Next, open the DHCP settings page on the ISP's router and reduce the allocated range. For eg: if the range allocated was from 192.168.1.1 to 192.168.1.253 change the ending address to 192.168.1.150. This is just an example, you can change the end address to anything you want.

After you have done this, go to your TC's internet settings and change the address allocation setting to Static; by default it would have been set to DHCP + NAT. Enter the IP address you reserved in the previous step, here. In all possibility it would already have be filled in.

Next, go to the DHCP page of the TC and change the allocation range to 192.168.1.151 - 192.168.1.250. We are allocating the remaining range to the TC so that it can hand it out to any connected clients.

Restart the ISP's router. Once that is up, restart the TC.
Put the time capsule in bridge mode: <https://discussions.apple.com/message/23393821> that turns off the 2nd router and let's your ISPs device perform router functions.