I connected my TC to my MAC through (join a wireless network) choice
This is a "wireless only" type of configuration. The ethernet ports are
not enabled on the Time Capsule in this type of configuration, so it is not possible to connect to the Time Capsule using any type of ethernet connection.
and I want to know if there is any way to switch between wifi and ethernet cab
Unfortunately, not easily. You have to "hard reset" the Time Capsule and completely reconfigure it to connect using ethernet. Then, you have to "hard reset" it again and reconfigure it back again to "join a wireless network". I would not advise most users to consider or attempt this.
IF...you have another router there providing your wireless network, and
IF...you can establish a
permanent ethernet connection between the wireless router and the Time Capsule,
We can suggest a setup that will give you the option to backup using wireless or using ethernet.
Post back if you want to consider this,or need more information.