With some good luck, the current settings on the Time Capsule might work with the "main modem" in the house without having to reset the Time Capsule to start all over again from scratch to configure the device.
We won't know how well.....or if.....this will work though until you try it out.
We assume that the Time Capsule is powered off at this time. If it's not, just pull the power cord from the back of the Capsule to power it down.
Connect an Ethernet cable from one of the Ethernet ports on the modem/router......(there are 4 of them on most products)....choose any one that you want, and connect the other end to the "O" Ethernet port on the Time Capsule
Power up the Time Capsule and give it a minute to run
Open AirPort Utility on your MacBook Air.......(Finder > Applications > Utilities > AirPort Utility)
Does the Time Capsule appear? If not, post back for more instructions.
If yes, click on the picture of the Time Capsule, then click Edit in the smaller window that appears.
If the light on the Time Capsule is flashing amber, look for the Status setting in the small window and click on the small amber dot there and post back with the message next to Status.
If the light is green, that is a good sign.
Either way, post back on your progress and we will continue.