want mobile app to ring when desk phone on DND

I don't think so. Instead you could either set your PC volume to 0 when you are away from the office.

2nd option is to quit the app. This means, for example if you are using Windows, at the bottom right next to the clock you can right click on the Ringcentral icon > Quit. Don't sign out of the app :D When you come back to the office, run the app and it will automatically sign you in.
If you do sign out, you would have to type password, and 2 factor authentication code.

Thank you for responding. To clarify: when I say desk phone I am referring to an actual phone on my desk. Not my computer. I'm not in the office right now so I don't remember the equipment model, but it's a desk phone provided by Ring Central, and it has a physical DND button.

Thank you for your help. I just want my mobile app to always ring regardless of what other devices are on DND, so custom rule is not necessary. I will check if the "active" slider is toggling off when desk phone is set to DND. Thanks for the tip!