VPN / OpenVPN behind a private WAN IP

I have a NAS server and a Raspberry PI running some services on my home network that I would like to be able to remotely access from my cellphone or my laptop while I'm traveling. The problem is that my router, which connects to the ISP's network via PPPoE, does not have a publicly available IP address. Asking the ISP to transfer port to my private IP address is out of the question.

I now have the feeling that OpenVPN should be able to solve this problem, but until now, I can not understand how. Do I need help from an intermediary to access my home network remotely? Should I install OpenVPN on all devices I want to access from the outside or a device with OpenVPN can it serve all others on the same network? enter the description of the image here