Several studies have been published that examine RIS as a substitute for amplifying and forward (AF) relays in order to achieve high-performance gains in the spectrum and energy efficiency of future wireless networks. Specifically, the authors of [17] demonstrated that RIS-aided transmission can outperform the DF relaying protocol in terms of energy efficiency (EE) for high-rate communications. However, the main advantage of RIS in the network is the algorithm structure for managing the phase changes of the passive elements. By creating the phase shift controller using an alternating algorithm design, the authors of [18] examined a RIS-assisted cognitive radio (CR) communication strategy for optimising the achievable secondary user (SU) rate of the system. Moreover, the authors of [6] analysed the RIS-assisted dual-function radar communications (DFRC) scheme for increasing the system’s secrecy rate through the use of a multi-eavesdropper, multi-cast, and multiantenna optimisation approach. Moreover, the performance of a RIS-aided single-cell wireless system with a multiantenna access point and multiple single-antenna users was examined by the authors in [10]. In particular, they focused on the asymptotic performance, or RIS, with an essentially unlimited number of elements and contrasted it with a benchmark enormous multi-input-multi-output (MIMO) system without RIS. Moreover, in [4], the authors developed an adaptative hybrid scheme technique that combined time domain techniques using reduced power Almost Blank Subframe (ABS) and power control techniques to mitigate the cross-tier interference between the femto base station and the macrocell user near the femtocell. Results obtained from the simulation demonstrated that the proposed technique could increase the spectral efficiency of femtocells. The technique proposed in this study only considers cross-tier interference between the femto base station and macrocell users. However, the energy efficiency of this technique was not considered as well. Additionally, in [19], the authors formulated a heuristic scheme called the Quality Efficient Femtocell Offloading Scheme (QEFOS), which classifies the users into three categories: macrocell users, which experience low cross-tier interference; macrocell users, which experience high cross-tier interference; and femtocell users. Nonetheless, the cross-tier interference in the case where the femto users experience high cross-tier interference and co-tier interference was not considered. Moreover, in [20], the authors proposed a method to mitigate downlink cross-tier interference between macrocells and small cells. The proposed method is based on an extended cross-tier interference mitigation scheme that coordinates not only cross-tier interference from macrocell to small cell users but also from small cell to macrocell users, which consists of mitigating cross-tier interference from small cell to macrocell users without changing the pilot allocation. Furthermore, the authors of [21] suggested a resource allocation scheme for RIS-assisted heterogeneous networks with non-orthogonal multiple access to improve spectrum efficiency and transmission rate. The proposed resource allocation scheme optimisation is based on the alternating iteration approach and successive convex approximations. Nevertheless, this study did not consider the performance of the proposed method with respect to the user’s location. Moreover, in [22], the authors investigated the performance of RIS-assisted wireless communication systems under co-channel interference in order to determine the closed-form expressions for the outage probability and channel capacity. However, this study did not actually take into consideration the RIS phase shift optimisation as well as the trade-off between transmit power and co-channel interference. Moreover, in [23], the authors developed a novel scheme for efficient multiple access in RIS-aided multi-user, multi-antenna systems. A comprehensive comparison of different schemes and configurations was presented in order to determine which scheme is better for the 6G paradigm. However, this study did not consider co-channel interference or inter-user interference. Moreover, the study in [24] presented a survey of the design and applications of an RIS for beyond 5G wireless networks. A comprehensive, detailed survey of RIS technology limitations in current research, related research opportunities, and possible solutions was presented. Moreover, the study in [16] compared the RIS-assisted system with the distributed antenna-aided system. However, the non-line-of-site link between the transmitter and receiver was not considered. In continuation of the same objective, in [25], the authors investigate the intelligent reflecting surface (IRS) for sum-rate maximisation in cognitive radio-enabled wireless powered communications networks. They proposed an alternating (AO)-based solution with a successive convex approximation (SCA) technique to solve the unconvex optimisation problem. The result of conventional cognitive radio enabled wireless-powered communications networks. Nevertheless, this study only considered cognitive radio-enabled wireless-powered communications networks. Apart from cognitive radio (CR), MIMO, and DFRC communications, RIS has been used in D2D and IoT communication systems. For example, in [5], the authors analysed the performance of a RIS-assisted IoT network using two resource allocation problems, namely, spectral efficiency and energy efficiency maximisation. In [7], the authors investigated an RIS-based unmanned aerial vehicle (UAV)-assisted non-orthogonal multiple access (NOMA) downlink scheme for maximising the sum rate of the system by using a deep deterministic policy gradient (DDPG) algorithm. However, except for the works in [19,20,21,22], where the authors proposed to mitigate cross-tier interference in HetNets without taking the impact of the users and RIS location in the network into consideration, a study of cross-tier interference mitigation in RIS-assisted HetNet is necessary, especially for maximising system sum rate and network performance with respect to user location and RIS location. Therefore, in contrast to the aforementioned studies, we examine an RIS-assisted HetNet where the reflected link from RIS to the macrocell users (MUEs), as well as the reflected interference link from SBS-RIS-MUEs, are employed to mitigate interference from small cells (SBS).