What's my option? Is there any way to sort the string like a number?
Yes, you have an option to sort string like a number, but in all your queries you need to convert your string field to an integer field just for sorting and it is not recommended.
Or I have to change my _id type?
Yes, you need to change the type of _id field from string to integer as a permanent solution. Since _id filed is an immutable field, you cannot directly convert the data of _id field from string to integer. you need to remove all your documents and insert with the corrected data type.