A ET-3850 was given to me and it would not air print. Reading this got me up and running in about 10 minutes.
Here are my suggestions.
At the printer.
Run a "Print Head Nozzle Check" this will. print out the serial number of the unit.
Goto Network status and print the Status Sheet" this will give you your IP address.
Mine was 10.0.0.80 this may be a default but I'm not sure.
This was the third line in the <TCP/IP IPv4> section.
With these two pieces of information you can access the Admin settings of the web tool.
Goto the a device that is on the same network with a web browser and enter in the IP Address as written.
This will open the web interface for the printer.
Click the Administrator Login button at the bottom of the list.
You will be prompted for a password and this will be your Serial Number. This will be listed as the first line on your
"Print Head Nozzle Check".
This will bring you back to the main menu, click "AirPrint Setup"
Click "Enable AirPrint" and "OK"
Wait a couple minutes fro everything to update.
If this was the issue you should now be good to go.