Technically it is possible to install the on-prem producer and connect this to a cloud Manager (back-end). However, it's not actually offered/supported, for a number of reasons. Also, as you note you need the installation files and these are only made available to customers who have purchased on-prem licenses. If you have a solid technical justification as to why you need to do this (e.g. very low-bandwidth or unreliable internet connection) ask your SAP CEE/CSP and see what they can tell you.
Sounds like you have SAP Enable Now Cloud and you want to install Producer, right?
You will need to install the App Launcher first. Navigate to Help > Download SAP Enable Now App Launcher and install it.
After that you should be able to see a link to Open Producer on the top right of the screen:
Click on that, and you will be prompted to save a file called Producer.senl. Save the file in your PC and that will kick off the installation.