You have made a mistake in the setup.
You do not "Join an existing networking" and use ethernet.. they are mutually exclusive.
What you are doing is backing up over double hop wireless which is slowing to a crawl.
You need to put the TC into bridge mode.. and plug it into the main router by ethernet. Then use wiretess to the TC if you are too far away or ethernet if you are close by..
As soon as you put the TC into join a wireless network it deactivates the ethernet.