From the User Manual of DGX 650, I can see that this keyboard does not have a built-in MIDI port, which is required to control the Ketron SD90 module. You will have to connect the DGX 650 to an external device such as the Kenton or MIDIPlus MIDI USB host to convert the USB port to MIDI in/out ports:

http://www.kentonuk.com/products/items/utilities/usb-host.shtml
https://www.amazon.com/midiplus-USB-MIDI-Host-Controller/dp/B074NGTGHN

I only tested the Kenton MIDI USB host by connecting a keyboard with USB to-host port to the USB 2.0 port of Kenton MIDI USB host, and Kenton's MIDI out port to the Ketron SD90 MIDI IN2 port - it works as expected using the default MIDI channel 1.

Thanks,
Paul