High Bandwidth Usage for secondary WAN



My setup is:
Peplink ONE with 2x ISP + 1x LTE-backup
–> connects through 2x SpeedFusion to my HQ

The LTE-Backup had to be realized through a second router on WAN3-port.

My problem:
If I set WAN3 to “Backup”, I cannot monitor the link and do not see, if the connection has a problem.
If I set WAN3 to “Always-On”, I see 1GB/month of SpeedFusion-Traffic, although WAN3 is configured with higher metric in the speedfusion-tunnels (500MB/month/speedfusion-tunnel).

Is there any possibility to lower this amount of traffic or to be able to “ping” over a backup-connection? If I use an LTE-Stick, pinging is possible for a “backup-connection” - for routed ports, it isn’t…

Thank you for your help!