Some tables don't have a recovery menu or an automated recovery procedure and because of that, the button combination will not work and it will just restart the tablet. In most of the cases, a simple reset can be done with the help of a Gmail account attached, but like you said, if there isn't one added, then this procedure is not available.
You will need to take the tablet to a local phone service shop and they will unlock it for you for a small fee. I recommend this way because they have the proper tools for this job.
