I'm a firm believer in not using any 'self-publication' at all.
It used to be called 'vanity press' and that's what it is.
You got two nice books. What will you do with them now?
If you got them to give to someone, that's great.
But obviously, you can't write your sequel through that company.
That's a big problem with self-publication. You can't do what you'd like to do.
I suggest you read the first section of a current "The Writer's Market" - "How to Publish." That section tells you all you need to know about how to publish properly.
It's an expensive book - if you can't afford it, suggest you study that section at your library. Make notes, copy pages on their free copier; know that section like the back of your hand.
In - "How to Publish" - you'll find tips on not only how to write a good query letter and how to find agents, editors and publishers, you'll find many tips by many people in the writing field - authors, agents, editors, publishers, screenwriters; many people. You'll also find "Predators and Editors" - a section to help you decide who is really a good editor or publisher and who is not.
Again, I strongly suggest you read that section first, follow the tips you find there and get published properly. Then use the book to find the correct agent, editor or publisher. You'll find your writing goes a lot farther and you're paid royalties (if accepted) as well.
Try it.
I have. It works.
Edit:
I don't know why Lulu is mentioned as a good vanity press. I've only read poor reports about Lulu. I usually read poor reports about most vanity presses. The only way a writer can make one work for them is if they have deep pockets (money to travel) and great associations with bookstore owners, book sellers, and if they're willing to wait quite some time for a 'real' publisher to take interest in what is written through vanity press and who will then buy copyrights of their own (through a contract).