The Garmin Running Dynamics Pod (RD Pod) does not work with Zwift Running. It doesn’t pair. I think DC Rainmaker discovered that the RD Pod doesn’t transmit pace.
I found out the hard way. 
Yup, did get the RD as well.
It does not get pace, just cadence.
But there is a trick, if you have a garmin or a nice mobile, you can download the “speed transmiter app” downloaded to your mobile and garmin (if the case) and you can use that as your pod directly in zwift. (Check the videos on youtube)
The problem is that you must pay for it as well after 8free running minutes.
It sucks…
Quick question Joao… Do you work for free?
Any how, the app mention is working very nice, used it my self some time ago, in manual mode, changed speed on treadmill and then in the app, some small bugs then, finished a group run doing 13km/h on treadmill but the app set my avatar at 30, thats a sprint 
#RunOn