Some people have had some issues with receiving the OTA updates after enrolling for a second time. This may or may not be the issue here as well.
To flash the Android N image manually, here is a good YouTube video that explains the whole process step-by-step:
https://www.youtube.com/watch?v=HhuCp7GY1OA
Also, as long as you stay enrolled, you will receive the next updates (and the final version whenever that comes out) OTA, so this would be a one time process.
EDIT: Although the video is for a 6P, is exactly the same process. The steps for enabling Fastboot, should you need them:
1. Power Down the Nexus 5X
2. When Off, Press and Hold the Power and Volume Down Buttons at the Same Time
3. Continue Holding These Two Buttons Until You See the Fastboot Mode Screen
4. Once in Fastboot Mode, Let Go of These Two Buttons
If you have any questions let me know!