I found this while searching its not from a Talk Talk forum just general discussion, as mine is off and the internet is working fine i guess that the way Talk Talk set them up.
If NAT is turned off, the device will work on pure-router mode which can transmit data only. Please DO NOT turn it off unless your ISP supports this mode, otherwise you will lose Internet connection.
Notice: The default status of NAT is Enabled, so without special demand, please don't select the Disable option