You have this:
<li><?php echo $retrieved_data->column_name;?></li>
<li><?php echo $retrieved_data->another_column_name;?></li>
<li><?php echo $retrieved_data->as_many_columns_as_you_have;?></li>
Here, the code tries to output the data in the column_name column, but there is no column in the table by that name.
Likewise, there is no another_column_name or as_many_columns_as_you_have columns in your table.
We know that the query pulls in the data, because when you used print_r we could see the data. We also know that when we did that we didn’t see fields named column_name, we saw fields named id and naam.
After a little digging, it looks like you took a generic example from here:
https://wordpress.stackexchange.com/a/54641/736
But didn’t make any modifications to match your table. The example wasn’t a copy paste work anywhere code block, such a thing does not exist for what you want.
Your table has id and naam columns, these are what the code should be using, not another_column_name or as_many_columns_as_you_have