Make sure you see the flashing blue light on the base of the rower. (The flashing indicates that the rower is in Bluetooth pairing mode). If you don't see the flashing blue light, then try to unplug and re-plug the rower so that the Bluetooth mechanism resets and the light flashes.
Go to a class, and tap the banner that says DISCONNECTED at the top of the screen. Your rower should show up in the devices list that pops up.
If this is not the case, then try to use the app on another device to see if the issue persists. If it does, then the culprit is most likely the Bluetooth mechanism in the rower, and that mechanism would need to be replaced. If it doesn't then the culprit is most likely the original device.