There are several possibilities from resetting the network connections to resetting the phone itself.
Make sure that the monitor isn't paired with the device (phone / tablet) itself. If you see it there, remove it in your bluetooth settings.
If you have the monitor paired with any other app other than the DDPY app, remove that as well since that could cause conflicts connecting to the DDPY app.
Here's a list of compatible monitors https://ddpyoga.zendesk.com/hc/en-us/articles/115003696607-What-Heart-Rate-Monitors-work-with-DDP-YOGA-NOW-