First thing first, did you set up your RFXtrx hub? And did you set up RFXtrx cover with automatic_add: true?
Which RFxtrx433 and firmware do you have?, not all of them support receiving commands from your remote according to the manual.
First thing I notice; ‘dummy: true’ should be ‘dummy: false’. ‘dummy: true’ creates a dummy rfx433 hub, which is for testing purposes. Can you try that first?
Next thing you can try is to see if it is automatically added. Press the remote and see if a cover is being added in the states menu in the development tools menu.