Transmission issue

Staying in second gear is "limp" mode that allows you to drive slowly to a repair shop without needing a tow truck. You can start the diagnosis by observing when it goes to limp mode. You get it out of limp mode by turning the ignition switch off, then restarting the engine. If you shift to "drive", and it immediately goes to second gear and stays there, the cause is almost always electrical in nature, to include a failed sensor or a problem with the wiring or connector terminals. If it starts out in first gear, then bangs back to second gear during or right after a shift, it is more likely to be caused by slippage inside one of the clutch packs. At the mileage you listed, either cause would not be uncommon.

Your mechanic will start by reading and recording the diagnostic fault codes with a scanner. Those will indicate the circuit or system that needs further diagnosis.