Complaints or orders issued by the manager shall be served upon persons either personally or by registered or certified mail, but if the whereabouts of such persons are unknown and the same cannot be ascertained by the manager in the exercise of reasonable diligence, the manager shall make an affidavit to that effect, and the serving of the complaint or order upon such person may be made by publication once at least 10 days prior to the date of the hearing in a newspaper having general circulation in the city. Where service is made by publication, a notice of the pending proceedings shall be posted in a conspicuous place on the premises affected by the complaint or order.