Because of the altitude, your engine can't develop the power it did at the near sea-level altitudes you used to fly at. You can try going with less pitch on the propeller, but you'll need to keep the RPM below 11,000. You'll still likely have somewhat lousy climb performance.

The only way to get the power "back" is to go with a larger engine. While the larger engine might be too big for sea-level flying, the reduced power level of the larger engine will be back in the range of power needed for the model.