The D-LINK 6850U is a VDSL modem + router given by Israeli telecom provider Bezeq to customers of their high-speed VDSL plans (so-called Next Generation Network - NGN). The device seems to be custom made by D-Link specifically for Bezeq, and it arrives flashed with custom Bezeq firmware that includes many “features” such as open-by-default remote management ports (with default usernames and passwords!) and remote “management” backdoors that are very hard to shut off.
Note on TR-069
The modem/router has TR-069 enabled by default, the hidden URL to manage it is: http://firstname.lastname@example.org (Where 10.0.0.138 is the default modem IP address).
There is not explicit “disable” button there but changing the “ACS URL” (Bezeq server) to some internal IP address like 10.0.0.199 should do it.