Use the manual script that Jdownloader provides.
I have ZXDSL 831CII and it was not supported by Jdownloader's "auto script". But Jdownloader also provides a manual script (reconnection recorder):
In their words:
"Many routers/cable modems have a web interface to config and control them. Normally, this web interface can be used to reconnect your router. JD's Reconnect recorder module can record while you are reconnecting your router with your web interface. Afterward, JD can replay this recording automatically so that you do not need to wait long periods between downloads from the same host."
Go to settings then under modules you will find reconnection. After that you will find 'create reconnection script'. The reconnection recorder window will show up. Press start and then your browser will open. Now perform a restart of your modem like you would normally and when you have done the restart, jdownloader will automatically notice your new ip address and a popup will come up saying that your reconnection was successful. Press yes to exit.
More detailed info at: