Alaeddin,
The UC320W needs reliable Internet service to operate correctly.
Based on your comment "
"I also noticed that if I try to access the web interface of the DSL modem almost never get through (sometimes rarely I can reach it)." The issue seems to be related to your DSL service provider. Did you try some extended pings from a PC directly connected to the DSL modem to the default gateway from the DSL provider? Make sure you have consistant reliable internet access and then reconnect the UC320W.
Regards,
Randy