The unexpected explosion of mobile data traffic has led to that the cellular networks are overloaded. Long term evolution (LTE) wireless local area networks (WLAN) aggregation (LWA) has recently emerged as a developed solution of mobile data offloading. In this paper, a multi-RAT heterogeneous network (HetNet) is considered, where three mode categories are defined, LTE mode, WiFi mode, and aggregation mode. A non-collocated LWA scenario is considered for the aggregation mode, in which a wireless backhaul link is connecting LTE with WLAN networks. We use the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) in order to arrange the available modes and their own stations, according to several metrics and based on the user requirements. Thus, helping for optimum mode and network selection that will improve the system performance while satisfying the user requirements. The analytical results show the importance of taking the effect of the backhaul during the mode selection process and how the backhaul link affects the overall performance.