mork
If the PPPoE username contains "-", the modem does not create a wan-connection.
You have absolutely right!
mork
After updating the firmware to version 1.02 and reset to factory settings, the modem works very unstable. The modem is working correctly the first few minutes, and then starts to reboot the ethernet ports and WiFi. The modem is not visible in the browser (page is not available).
Which devices have you connected via LAN cable to the 2750B? Are they connected through an external switch?
mork
The modem is stable only on firmware v1.01.
Have you tried the firmware version EU 1.02 20130610?
P.S. Belarus is under D-Link RU and they have the russian DSL-2750U not the european DSL-2750B.

Have you bought your device in Germany? If you are entitled to the German D-Link Guarantee you can contact the Germany Support to try to have a new firmware stable release. D-Link Germany is the most important european D-Link subsidiary and probably they could have a new firmware realease faster than the others.