Version 6.0 and above
To disable the responsive layout you can use the same number of columns on Mobile as in the Desktop configuration. This can be changed navigating to Instagram feed > All Feeds > Feed Settings (pencil icon) > Customize tab > Feed Layout > Columns.
Below version 6.0
If you’re using the latest version of the plugin then you can disable the mobile layout by checking the Disable mobile layout setting on the plugin’s Customize page. This will then maintain your multi-column layout on mobile devices rather than changing to a one-column layout.