how to set up custom domain for a website on a heroku-hosted locomotive

Hi,

I just deployed my locomotive app successfully to heroku (using mongoDB Atlas instead of mLab since it's not available anymore).
I was wondering how to set up a custom domain for a specific website and I found some answers here, but unfortunatly it uses a IP redirection to work, and it seems to me that free heroku-hosted app have no static IP addresses. So the proposed solution does not seem to work in that case.

Am I wrong ? If so, can you help me ?

Thanks,

Grégoire