You can't use both USB and MIDI DIN at the same time.

However, if you connect the Montage/MODX to a computer via USB, in the MODX MIDI Ports list in any software you'll see 3 Ports.
Port 1 is for working with the MODX directly, Port 2 is for DAW control and Port 3 is for addressing whatever is connected to the MIDI DIN of the MODX.
If Bandhelper does MIDI routing, you can route MIDI from Port 1 to Port 3 (this is what I do in Cubase/Nuendo when I want to work with the DAW and MIDI devices at the same time).

That's exactly how to get the job done. If bandhelper doesn't display the port choices you may be able to find a virtual MIDI cable application that can provide this.

I don't use BandHelper but this tutorial has wording that suggests you can select the port you want to use.
https://www.bandhelper.com/tutorials/sending_MIDI.html

To register your MIDI devices with BandHelper

Before sending MIDI to a device, you must add the device to your account. This allows you to see your MIDI device names in the app and quickly change the port or channel a device uses without having to edit all your MIDI data.
. . .
You can also set the port, but it's best to leave that set to "All" unless you have a specific need to target one port (such as if different devices on different ports of a multi-port MIDI interface need to use the same channel).

There is a 'port' selection in the graphic on that page with a default port of 'All' and a dropdown to choose another port.

I honestly don't know if that is the type of port support the others were referring to or not but you might want to check it out.

I have Bandhelper iOS but there's also a MacOS version and Android version. Eventually you may want to share which flavor you use