Thanks for clarifying. You will need ACF Pro to add a custom numeric field to a post or image attachment. Just in case you don't know, ACF Pro is bundled in our theme. See https://theme.co/apex/forum/t/integrated-plugins-acf-pro/49
To add a custom field to Media Attachments, please read this documentation: https://www.advancedcustomfields.com/resources/adding-fields-media-attachments/. The field setup should look like the screenshot below. Take note of the Name and Type.
At this stage, you now have a custom field labeled Order when you open an image in the Media Library.
Now in your grid Ordering settings, choose By number meta value under Order By and insert the ACF Field Name in the Order by a meta key value field.
In case you can't get that to work, please give us WordPress Admin access in a Secure Note as previously requested.