New solid-state sources of Gigahertz-Terahertz electromagnetic radiation continue to have many applications in high-speed electronics, communications, security, and medicine. To develop new devices, it is important to understand the coupling of such high-frequency sources not only with each other but also with their environment e.g. to achieve increased power output, synchronization, and to control interference. Using Graphene-based materials is particularly promising due to its high electron mobility and configurability of the device structures. However, accurately modelling its electromagnetic behavior computationally along with the inherent complexities of the device itself (e.g. non-linearities, and quantum effects) can be quite challenging using current tools. To address this, we use a simplified approximate model to reduce the complexity of the structure and derive new formulations that describe its electromagnetic and intrinsic behaviours. In this poster, we report new formulations, finite element spaces and general progress in modelling a graphene hexagonal-boron-nitride resonant tunneling diode (GRTD) using the time-domain boundary element method. We also explore the possibility of mutual coupling and synchronization between two GRTD devices as well as their radiation patterns and total output power.