Renault Laguna "Falla"

Good afternoon, I have an Rxe 2.0 lagoon for a couple of weeks, I bought it with a fault and I would like to get it out. The engine only works with CNG since it does not have the fuel pump. When starting the engine it costs a lot and when I manage to start it, it has no strength until it passes the 2,000 reollations, what is the fault? Thanks to those who answer.