Hi,
You should able to manage the BR1 device (IP passthrough enable) from the LAN site.
Scenario 1:
IP Passthrough with Router connected to BR1 LAN
BR1 IP passthrough Cellular WAN
Router WAN IP
PC that connected to router able to manage BR1 by using IP address 10.120.12.186.
Scenario 2:
PC direct connect to BR1 LAN interface
BR1 IP passthrough Cellular WAN
PC received DHCP IP from cellular network
**PC that direct connected to BR1 able to manage BR1 by using IP address 10.120.12.186.
**
Thank You