Telnet isn't enabled by default in Win 7, if that is what you are using and you haven't already enabled it.
To enable Telnet in Win 7 go into Programs and Features and click on Turn Windows features on or off then scroll down and check the box for Telnet Client - OK - and then you log into Telnet the same way as you would the 7800N except that you press Enter after admin and your password.
From Start type
telnet 192.168.1.254 then press Enter.
Type
admin - press Enter and then your password at the next prompt.
If you make a typo then you will have to start again with
admin.
It's been a little while since I've entered the next command and I don't have my 7800N hooked up at the moment, but I think the command is
adsl stats --info to display the stats and there is a space before the two dashes.
(Not yet senile but the memory can be problematic when I haven't done something for a while

)
It will be the errors for the last 15 mins that you will be interested in when the speed has dropped through the floor.
You probably know how to do this, but right click in the text area and click on
Select all to highlight the text then press Enter.
You can then copy that to Notepad and then copy & paste that into the Reply box.