Hi,
is there a way where both can work together?
This may not be possible because when the responsive image option is enabled, WP will automatically select the appropriate size based on the current screen resolution instead of the selected thumbnail. Did you refresh the site or purge the cache before regenerating the images? Please make sure that the site is using the latest version of the theme, 4.7.6.4.
Would you mind providing the WP and FTP login details again so that we could test it properly?
Best regards,
Ismael