Hi and welcome to Digi Forums!
The functionality you request would need to implemented via a script. Do you mean Eth WAN and not LAN? If you did mean LAN then changes references from wan to lan where found.
System > Scheduled Tasks > Custom Scripts > add a new script.
Name = Disable_modem_if_Eth_WAN_UP
Run mode = Interval
Interval = 5m # This is 5 mins, reduce if needed
Run single = Enabled
Script to use
# disable the cellular modem connection if WAN is up
# enable the cellular modem connection if WAN is down
wan_iface_state="$(runt get network.interface.$wan_iface_name.ipv4.up)"
case "$wan_iface_state" in
1) # up
config set network.interface.$modem_iface_name.enable false
0) # down
config set network.interface.$modem_iface_name.enable true
*) # unknown state
accns_log w config "custom test: unknown state for $wan_iface_name interface ($wan_iface_state)"
end of script
Log script output = Enabled
Log script errors = Enabled
Potentially you'll also need this script to monitor the Eth WAN interface, but that will depend on your network topology. http://ftp1.digi.com/support/documentation/accelerated/config_examples/WAN_Cont_Test_Failover.pdf
For any further help or investigation on that issue, please review our support options on our website: https://www.digi.com/support and mail our support team at firstname.lastname@example.org
Digi Technical Support Engineer