Not sure I understand. The PLAY:5 is not a Sub, it can not be added as one, only a normal speaker. 

If you can’t add it to your existing system using the App’s add function go ahead and hook it to Ethernet and do the setup process to add it as a New Room.

It should be working at that point. Next wait a while for everything to sync and remove the Ethernet, it should continue to work.

If it fails hook the Ethernet back up, wait five minutes (for data collection) and check that it is working. Then submit a diagnostic and contact Sonos Support to have them check that the radio is working properly.

But it will never be capable of being added as a Subwoofer, that isn’t its function.