You will also need to dump all the cached DNS data stored by the Modem, the Router and you computer,
In the following order:
#1: Remove the AC power from the Modem at the power brick
#2: Remove the AC power from the Router at the wall receptacle.
#3: open a command prompt (Start/run/ enter CMD)
At the open command prompt enter the following command:
#4: Power up the modem and let it boot up completely before proceeding to the next step.
#5: Power up the Router and let it also boot up completely before proceeding.
#6: Open a browser and enter your Routers setup pages and verify the following settings are in effect:
That the Router has been instructed to use a specified DNS service
And the primary and secondary DNS values have been entered.
As to the 11.2.1 error, what was the duration and number of transitions ?
Most errors are very transitory in nature.
At the time that the "internet was down" what were the weather conditions at your location ?
Just as important what were the weather conditions at the location of your Gateway ?
What troubleshooting steps did you take and what errors are you getting that leads you to think that DNS is at the root of your issue ? Can you post a screenshot ?