The high reliance on the Internet observed recently makes the quests of mobile networks capacity demand and reliability crucial in mobile networks design and management. The next generation mobile networks is anticipated to address the problem of increasing capacity demand by employing various techniques such as Massive MIMO, Heterogeneous Networks (HetNets), and millimeter wave (mm-Wave). Nevertheless, the benefits of the next mobile networks technology can be realized by network operators and end users if proper network design is well investigated. Network equipment failures are likely to occur, thereby, causing a remarkable monetary loss to both network operators and end users. In addition to the reliability problem, proper network design can reduce capital expenditure (CapEx) and operational expenditure (OpEx). In this study, we investigate the problem of mobile networks design for the next generation mobile networks. We consider two nodes namely Optical Network Unit (ONUs) and Base Stations (BSs). The problem is improving network capacity and reliability through adding new BSs to the live network by connecting to the ONU with optimal OpEx subject to CapEx and reliability. We present an Integer Linear Program (ILP) model that addresses the problem. Finally, we present numerical results that show potential benefits of our study.