Okay, I've figured it out. I'll put the answer here in case anyone else has the same issue. I guess I didn't understand the instruction in the Installation guide until I ran into the problem myself!
It's resolved using the 'manual' value in the shortcode
(manual: Each letter will create the begining of a new column. Exmple: 'manual="e, h, t".' This will create four columns (a-d, e-g, h-s and t-z+numbers). Writing "a, e, h, t" will have the same effect. The "columns" option is ignored as the "manual" setting creates the columns based on your input.)