If you have the Time Capsule on Bridge Mode now......then that tells us that you have another router upstream on the network that is providing IP addresses to all devices on the network.
That router or modem/router......you might call it your "modem".......is the device that you would want to configure to provide fixed or static IP addresses to devices on your network.......not the Time Capsule.
You would only use the DHCP Only mode if your ISP was providing you with a fixed bank of Public IP addresses to use. That is very rare these days. Or, you have a separate DHCP server that can allocate addresses to the Time Capsule. This gets complicated.
If you really do want to try to have two routers on the same network both competing with each other......not recommended......then you would use the DHCP and NAT setting on the Time Capsule. If you decide to try this, that is going to complicate things quite a bit on your network, and you will have to configure everything to connect to the Time Capsule and click to ignore a message that will appear that tells you that the Time Capsule should be in Bridge Mode.
The best....and simplest thing to do would be to ask the support folks for your modem/router if the device can be configured to provide fixed or static IP addresses to network devices. Assuming that it can, your next question would be "how" to do this.
