In this paper, the authors present a proof-of concept demonstrator for an adaptive inter-system-interface gateway between LMR and IP based networks. To underline the flexibility of the proposed solution, a satellite link is utilized to enable the integration of an independent LMR cell or direct connected terminals (MicroSpot) into an existing infrastructure over long distances without the need for dedicated transport networks. This set-up is motivated by both, homeland security operations as well as industrial use-cases. The scenario consists of an isolated communication group which needs to be linked with a core network from a degraded area. To deploy such a system, a prior in depth analysis of the restrictions of the components is performed. These results are used for an overall usability analysis based on measurements with a demonstrator. Main challenge is a high variance in the delay, which inhibit a direct connection between a strictly timed TDMA LMR network and IP-based transport networks. The proposed gateway is based on an adaptive convergence layer multiplexer, which optimizes the LMR data stream for the transmission on a time variant channel like a satellite link. This enables homeland security organizations to operate independent from land based radio networks.