Can I control Symphony Desktop’s settings from an iOS device (iPad or iPhone)? 

No, Symphony Desktop’s settings can only be controlled from the touchscreen controls on the unit or with the Symphony Desktop Control app on a Mac or Windows 10 PC.

The Symphony Desktop Control App for Mac and Windows 10 is available with the 1.19 version and up. Symphony Desktop v1.19 Release: Update Announcement and Notes

https://apogeedigital.com/support/symphony-desktop#schedule