A good R2 score but the model not performing well on testing data may be an indicator of overfitting. You can try the below mentioned steps to improve the model performance.
- Train the model on more features/predictors.
- Data augmentation to generate more training data.
- Regularization.
You can refer to the link below to understand more about regularization:
To understand more about overfitting and avoiding it, you can refer to the link below
