mobile apps: restful api or sdk dynamodb

I'm building my first app which uses an external database for storing data..

And I'm struggling with the following question: How to connect with my database: use the Amazon DynamoDB sdk for Android or create a restful api based on NodeJS on my own server, which passes the data to DynamoDB?

I'm very new to this and when I look over the internet I see amazon suggesting to use the SDK.

The only problem is, if I want to change the data structure in the feature, what kind of problems will my users experience with the current versions of the app? And is it safe to save my Amazon keys inside the app (cause people may be able to decompile the app)?

On the other hand, I do have to pay for the extra server which handles the connections between my app and DynamoDB. So... is it worth it?

So I'm quite struggling with this.... What do you guys think?