E398 - built-in idle timeout?

[Guest EmleyMoor]

I am using an E398 to provide a mobile connection to a Linux based router, as a backup to the main VDSL connection.

I have found that the connection, if left idle for around 30 seconds (exact time varies) hangs up, seemingly as though, to each end's perspective, the other end did it.

I am wondering, therefore, if there is some kind of idle timeout built into this device, and if so, how to disable, or at worst lengthen it.

The device starts to be less reliable at connecting after a few connection attempts. Being able to leave it connected would mitigate this.