SVM-Multiclass Classification using weights and bias

I've used Matlab Classification Learner App to train my SVM classifier and i have 99.9% of accuracy in prediction (i tested it with the function predict on matlab). What i wanted to do now was to predict without usind this function but using the hyperplane. I exported the trained classifier and so i have all the weights and the bias to find the hyperplane. Which formula should i use to predict new data? I tryed computing the sign of w'x+bias but it works only in few cases. Can you help me understand what should i do?

Thanks a lot!!