Connect Strapi with a Javascript frontend

I’ve created a website (vanilla JS/CSS/HTML) which uses Strapi as a backend. It works fine on my localhost. But I cannot get it to work on the DO platform. I’ve uploaded both of them as apps, but I can’t get my static website to talk to / access the Strapi backend.

The two apps work independently; I can log into Strapi and create/modify data and I can visit my website and see the various pages (but no data) so they’re working, but not communicating with each other.

I’ve gone through the relevant tutorials but can’t see what I’m doing wrong. Any and all help greatly appreciated - if someone has done this before, please point me to them. Note this is not React or Vue - it’s just plain old vanilla Javascript.