@PerK 

Hi, thank you very much for posting on TP-Link Community.

1. Is your router bound to your TP-Link ID? Please make sure the same TP-Link ID is used to login the Tether App.

2. When you are at home, please disable Wi-Fi on your phone, use mobile data only, then check if the router shows offline on Tether App, if so, use another client device to check if the router is really connected to the internet or not.

3. Please reboot the MR400 router, then check if it will be online on Tether App via mobile data. If so, please check later after several minutes to see if it goes offline again.