If a timely petition for reassessment is filed and after any decision on the issue of liability arising under section 23-724 affecting the assessment has become final, the department shall reconsider the assessment and render a decision. The department may increase or decrease the amount of any assessment under review. The decision of the department becomes final with respect to the employer and the lien imposed by section 23-745 attaches unless within fifteen days after written notice is served personally or sent by certified mail to the employer’s last known address the employer files with the department a written request for review by the appeals board with payment of the amount assessed.