I've never heard of a primitive road before.
Did the roadway you received your citation on have a NAME to it? If so, your regular Sec of State [ SOS ] licensed plate would have covered you LEGALLY being there. If it was a forest trail or forest road [no name to the roadway] that was NOT posted as being closed, or was not a section of National Forest land as posted closed, your SOS plate should have covered your operating there as LEGAL.
By chance, you were not operating in a State Game area that was posted as closed were you?