Hello, I wanted to calculate RMSE between two DTMs. How can I do that?

I don't know about RMSE but to get the difference in height you can convert one to Pointcloud and use PointCloudOnRasterComponentSetter, create a custom component and use it to subtract component z in PointCloudExpressionEvaluator.

Now use PointCloudStatisticsCalculator on the resulting component to get Stdev, Median, Mode or coerce to individual points and use StatisticsCalculator.