Doorbell out of range

Hello everyone
For a few days now, the doorbell can no longer be reached by mobile phone. There is no connection on either phone (out of range). I've already reset the bell, replaced the batteries, restarted both phones and turned the Bluetooth connection on and off. Removing the doorbell from the app is also not possible, because no connection is established. What else can I do? It worked perfectly until a few weeks ago.
Greetings Frank