Exterior Shower

What exactly is to short?

Do you mean the hose is too short? It should be possible to remove the hose and install a longer one, though exactly how to do this would depend on the exact fittings used. At the worst, you could cut the hose near one end and splice in some additional length (with the appropriate hose fittings/connectors, which I'd think would be available at home centers, hardware stores, etc.).

Or buy a trailer compatible with the new shower

Buy the right shower