Indoor cycle training & using a HRM-Dual with my Fenix 6x

I have recently restarted indoor cycle training using the Rouvy app. This app now sends all the required training data to Garmin so I have no need to record this only watch or Edge 1040 to update the training scores etc.

The set up I generally use for cycling is to send my WRH data to my Edge. I know it's not perfect but I couldn't be bothered to use the chest trap that I have. Recently Ive had some issues with the WHR not reading anything for periods as well as nonsense readying (way too low). 

I have now charted using the strap and send my HRM data to direct to Rouvy as well as display it on my Edge (easier to see). Ive noticed that although my ride data is correct the heart rate chart for the day is talking the data from the Fenix WHR and shows a lower amount and the drops when the Fenix stopped recording. 

Ive tried connecting the HRM to the watch but it doesn't seem to stay connected if Im not recording an activity on it.

Im guessing this is by design and unless I record the indoor ride on the Fenix (or edge?) as well then there is nothing I can do?

Please advise?