[Fixed]-Manager isn't available while setting up django-fmc

swappable is an undocumented feature, actually only supposed to be used for custom User models. The doc on custom user models clearly states that once you use a custom user model, directly referencing contrib.auth.models.User won’t work:

If you reference User directly (for example, by referring to it in a foreign key), your code will not work in projects where the AUTH_USER_MODEL setting has been changed to a different user model.

You probably want to read the rest of this chapter FWIW.

To make a long story short: as Daniel Roseman mentions, you very probably want to use your own MyDevice model instead of the default Device one. And eventually contribute back a patch to django-fcm doc if it solves the issue.