After upgrading my RUTX50 to RUTX_R_00.07.15, the router has started to drop the mobile connectivity, maybe once per day. The connectivity is only restored after rebooting the device. The issue is definitely in the firmware, because the LTE connectivity was rock solid in the older firmware. The LTE connection and subscription are fine; the issue is in the router itself. According to the event logs, the IPv6 and IPV4 connectivity just drops without a reason.
I highly request that Teltonika take the issue seriously. There must be others as well with the same issue.