Putting this in as an answer instead of just a comment. Sorry for the duplication.
I've received two separate responses from Mathworks:
1 - There is an outage for a number of support packages and they're working on a resolution
2- for MinGW Ad-On for 4.9.2 specifically:
We have been running into issues with the MinGW Add-On for 4.9.2 that our developers are currently investigating.
One possible workaround to the MinGW installation issue is to manually download MinGW, and link MATLAB to the installation with an environment variable. To do so, download and install MinGW from the following location: https://sourceforge.net/projects/tdm-gcc/files/TDM-GCC%20Installer/Previous/1.1309.0/tdm64-gcc-4.9.2.exe/download
Be sure to uncheck "Check for updated files on the TDM-GCC server" when running the installer to insure version 4.9.2 is installed.
Once the installation has finished, create an environment variable named MW_MINGW64_LOC, and set the location to the folder where MinGW was installed. For example, if you chose to install at the default C:\TDM-GCC-64 location, you will set the variable to "C:\TDM-GCC-64".
To set the environment variable, open the Control Panel, and go through the options System > Advanced System Settings > Advanced tab > Environment Variables. Be sure to declare the MW_MINGW64_LOC variable in the System Variables section rather than the User Variables section. After setting the variable, hit OK and Apply on all menus to have the variable assigned.
When complete, you should be able to start MATLAB and run "mex -setup" and have the MinGW compiler recognized
These instructions worked for me.