said by DoC_DaR :

This is the procedure that works for me.

Siemens Speedstream 4200 Bridged Configuration
1. Open your web browser and go to »192.168.254.254/. Log in using admin as
the username and using admin as the password. You will be presented with the PPP
Setup page. Completely ignore all the settings on this PPP Setup page.
2. Click on the Setup item along the left-hand side of the page. A submenu will
open displaying detailed configuration pages. Locate and click on the WAN Interface
item.
3. Click on the uppermost Delete button to delete all circuits shown on the WAN
Interface area. Once everything is deleted, it should look like this.
4. Click on the Add New VC button to add a new circuit.
5. Change the VPI to 0 and change the VCI to 35. Set the Encapsulation Type to
LLC. Click on the Next button to continue.
6. Select RFC-2684 Bridged as the Protocol. Click on the Finish button.
7. Locate and click on the DHCP item beneath the Setup menu.
8. Click on the Disabled radio button and then click on the Save Settings button
near the bottom.
DHCP Screenshot
9. A notice will be displayed reminding you to reboot the modem. Go ahead and
click on the Reboot button to reboot now.