Complaints or orders issued by the County Manager shall be served upon persons either personally or by first class mail, but if the whereabouts of the persons are unknown and the same cannot be ascertained by the County Manager or designee, in the exercise of reasonable diligence, the County Manager or designee shall make an affidavit to that effect, and the serving of the complaint or order upon the person may be made by publication in a newspaper having general circulation in the county at least once, no later than the time at which personal service is required under § 91.39. 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.