Hello,
You can follow the answer I’ve provided here: Alerts & Automations | Device Status Change - #3 by MatasR
If the router loses connection to the internet, it will automatically become offline in RMS, meaning it will send you an email (or whatever you pick) as a notification.
Otherwise, you can try using Events reporting, however, do note that it’ll be removed with future firmware releases: RUTX50 Events Reporting - Teltonika Networks Wiki
Regards,
M.