Forum member @wizard has just created a DNS utility named "DNS Changer" that sets the DNS server of your choice. It has a menu entry in the Network menu that launches the program to input your DNS server in a window and retains your chosen DNS servers after rebooting. It is a pet file - so to use in FatDog it needs to be manually extracted and the files copied to FatDog's file system. Tested on FatDog64-903.
Thread with utility is here --> viewtopic.php?p=136079#p136079