Time capsule reconnect wout ethernet

Hi

I used connect my modem to TC and use its wifi. After a period of not using TC, I want to reconnect it to my iMac for the backup purposes only. I already have a wifi IMac uses and I don't intent to connect TC to the modem via cable ( logistic problems) When I powered TC and see the old wireless name under network listings and connected to it but when I tried to connect to TC it doesn't, thus no backup. How can I set it so that I can set it up to work with the new modem/wifi without any cabling? I guess I first need to connect to it to do a setup. The light on it is yellow and blinking. And yes I really don't understand how this stuff works 😟

Thank you 🙂

Ceyda