My immediate thought is that this is probably being called outside of the loop, which means that you’ll need to add an extra parameter to the have_rows() to also reference the Post ID, so it knows which post to look for the information in.
In practice this would look like the following:
<?php
global $post;
if( have_rows('testmon', $post->ID ) ):
// loop through the rows of data
while ( have_rows('testmon', $post->ID ) ) : the_row();
// display a sub field value
the_sub_field('imagetes');
the_sub_field('desctes');
the_sub_field('avttes');
endwhile;
else :
// no rows found
endif;
?>
