I have 2 MXs, and internet link and a MG21 cellular gateway.  I've configured the internet (WAN1) as the primary link and load balancing is disabled.  I still see some traffic on the backup link via the MG21 cellular gateway, more than I was expecting. 
 
Is there anyway to reduce the 'keepalive' traffic on the backup link? 
 
Note: I don't mind increasing failover times (within reason), if I reduce cellular costs.