One person used the restroom and the toilet filled with water to the brim and then flushed regularly. A 2nd person used the same bathroom and when they flushed the toilet filled again and stayed in the bowl. We have planged vigorously and have used a whole bottle of drano max gel and still cloged. What could be wrong and what can we do to fix it.

The clog may be too large, strong, etc. Try using a manually worked snake or electric one.