If you are sure your’re seeing data from your devices (and not some other devices in your neighbourhood), then apparently they (re-)joined to the other network. If two networks know the same device, then both might receive the Join Request and might create a Join Accept, in which case it will be a roulette game as to which Join Accept is handled by the device, if any. So, make sure they’re no longer registered with the other network (at least not with the very same values for DevEUI, AppKey and AppEUI).
Indeed, you will need the DevEUI and AppKey (in the device’s settings) and the AppEUI (in the application’s settings; in V2 make sure to select the right AppEUI in the device’s settings, in V3 you will also be able to set this in the device directly). If the devices already have those assigned/programmed, then you can also copy these existing values from the device into TTN. For the AppEUI you can add one or more AppEUIs to an application, using either TTN Console or the command line
(Like written above: when not using a TTN-generated AppEUI then make sure the other network does not know the device with the very same values.)
I asked as sometimes TTN Console does not show data, while the other interfaces work just fine. Something to remember when troubleshooting. Also note that the MQTT API has an error topic, which (sometimes) can help during debugging. So even a command line MQTT client can be very helpful.