Let me just explain the situation I am in:
1. Basically I am in charge of spear-heading my company’s social media.
2. Instagram, AFAIK, does not have any native web-browser based features that allow us to schedule posts or even upload posts.
3. I need some kind of free-to-use tools / software that will allow me to schedule posts to Instagram and twitter. The only ones I’ve seen so far with a 10 minute google search charge a monthly fee to do this.
Use Later.com for Instagram. It’s free for basic features like scheduling posts. If you want to schedule stories, that’s a premium feature you have to upgrade to.
Later allows you to prepare everything for your post such as description, hashtags, location, etc.
I don’t remember if you can schedule carousel posts on the basic plan, but you can at least do single photos or videos.
Edit: As to Twitter, you may be able to share your Instagram posts to twitter unless you want them separate for reasons of separate metrics tracking.