For those who do not want to enable Telnet to adjust the SNRM can use this alternative method.
Save a copy of your Fritzbox settings.
Here is how to do this. You will need to assign a password to the file for saving and restoring it.
A export file will be saved to your PC.
You can open this file with Notepad.
Once opened, CTRL + F to bring up the search function and search for ( DownstreamMarginOffset )
You should then see the line ( DownstreamMarginOffset = 0 ; )
-10 to -100 is to reduce your SNRM for increased speed and 0 to 100 is to increase the SNRM for Stability.
Save your changes.
Here is the screen to restore the Config file.