Leider kann man die Setting URL: nicht mehr ändern ist grau....
You wrote "Unfortunately, one can change the setting URL: no more is gray .." this would indicate that the 870 is either a) getting its settings from provisioning and needs to be factory reset or b) the 870 is being redirected to obtain its settings from a provisioning server.
Try factory resetting the phone
1) Press **## on the keypad, press and hold the # key immediately afterward for about 15 - 30 seconds
2) The phone will boot into recovery mode.
3) Choose option 1 when prompted to default the phone to factory settings
4) Press any key when prompted
If the settings URL is still grayed out then the phone is obtaining its settings via auto-provisioning (DHCP, PnP, Redirection)