Apologies for the late reply.
I misunderstood, what was meant by the "bridge".
In the LAN network, it is a virtual interface, which connects multiple other interfaces, by extend broadcast domain from a single interface to multiple housed under it. Usually this is used to keep wireless and LAN networks within the same domain, in order for all the devices to be within the same subnet and firewall zone, get IPs from the same DHCP.
In regards to mobile interface, "bridge" has another meaning, and that is what I thought of. When configured in this mode, mobile interface passes IP address assigned by ISP to the first connected interface, or interface, which has its MAC address defined in mobile interface settings of TRB140. In this mode, most of the device's functionalities, including firewalling are disabled, and basically it operates as a transparent gateway.
In this case OpenVPN client would have to be the device behind TRB140.
Based on your current configuration and firewall settings, I do not see, what could be preventing NSAP transfer, if it is carried over IP protocol.
If there are specific ports, used by your device to tunnel this, packet capture with TCPdump utility can be attempted to see, if intended packets reach TRB.