Recently I was trying to set up auto posting for my website Android Beginner Developer Help. After doing some research this is how I did it.
First install wp-o-matic from your wordpress site dashboard.
After installation it will come up in settings.
Select add new campaign, you will need a feed URL to insert into it. Add name and save it.
Submit it and if feed URL is OK it will create a new campaign.
Now go to options and see the cron URL.
Rest all you need is to go to site like setcronjob.com and set a online cron job using the URL.
See my website Android Beginner Developer Help to see wp-o-matic in action.