So, the manual says "The driver implements BACnet/IP over UDP. Any access to devices on other network media types (Ethernet (ISO-8802-3), MSTP, ARCNET, etc…) must be done through a gateway/router device."
I have BACnet/IP access to a gateway device that connects to the other network media types. But I don't know how to set up the OPC paths properly to access things that aren't on the local device. The manual seems to only cover accessing a point on the thing you're connecting directly to. How do I address a "remote device" when the BACnet/ip device I'm connected to is a gateway?
That's not really how it works.
The gateway is responsible for routing traffic between the subnets or other network types. You may have to configure the BBMD settings on the Local Device to point at this gateway.
You don't address other devices through a remote connection to this gateway. Each remote device will be its own device entry. These entries will probably point at the IP of the gateway and then you need to know the remote device id you're trying to communicate with. The gateway routes traffic to/from a target remote device on a remote network.
Bacnet IP is the key, if you have Bacnet Ethernet and MSTP, they have to have a path to the Bacnet/IP devices. And the Bacnet IP Device needs to be a router. You would then configure your device instance in ignition to use the routing devices IP and then hit the bacnet address of the non IP device. If your IP devices are not router type then you will need a gateway like a Fieldserer to translate from Eth or MSTP to IP