Is the router also resetting without any changes? If yes, modem is not ok. Ask KPN for a new modem.
If no, check your changes.
Follow this procedure to make the changes in your 780 router.
First
set "Always use the same address:" for your server ip-number/device name.
Second,
select "Game & Application Sharing" in the toolbox menu.
select "Assign a game or application to a local network device"
select "FTP-server" in the "Game or Application" drop-down menu and your "device-name" in the "Device" drop-down menu.
click on [add] button
Third,
select "Game & Application Sharing" in the toolbox menu.
select "Assign a game or application to a local network device"
select "HTTPS Server" in the "Game or Application" drop-down menu and your "device-name" in the "Device" drop-down menu.
click on [add] button
Now must it work. (I hope
)
Good luck!!!