What updates did you do a month ago??
The latest firmware for the TC certainly seems to have caused issues..
Try a full factory reset.. using the latest firmware.. if the issue continues.. go back to 7.5.2 and again do a full factory reset and see how it goes.
Also see if you can test from another computer... and use ethernet as well as wireless.
Can you also take the TC completely out of the network.. turn it off.. just link by wireless to the dsl modem.. you seem to suggest that is very slow.. is it still??
If so a driver update from Lion more than likely has caused issues with the wireless card.
Try ethernet and see if the dsl is actually slow.
If you can narrow it down to the wireless in Lion.. I would ring Apple and explain what happened.
I would use a different method to connect.. wireless bridging router to ethernet on the computer or something that bypasses the issue.
I can't quite follow your post:
If you connect the TC (in default settings) to one of the ethernet ports on your modem/router, you see a new WiFi net on your computer, connect to that one.
You might need to do a reset of your modem and the TC: disconnect both from the power, after 1 minute reconnect the modem to the power, let it come online, then reconnect the TC to the power, let it come online. Maybe you restart your computer also. Do you not see the new net? Is it slow?
If so set the TC to a different channel.
LaPastenague, Thank you for your suggestion.
Actually I made that update on the firmware! You think this could be the cause of the problem?
Because I already tried everything you suggested, including disabling wireless, trying from another MacBook, etc..
One more thing is annoying me today: any time I try to perform a backup with Time Machine with the current settings it fails, raising a kind of error which says that is not possible from the Mac to crate the folders for the backup...
However, today I'll try to go back to the former firmawere version... Will see...