Let me help you with this confounding issue on your Proxmox server. I have over 8 years of experience in system administration and configuring Linux environments, and I can confidently handle your case.
Besides my expertise in Linux-based systems, I also specialize in control panels like Plesk which means I'm well equipped to solve the internet connectivity problem your Debian CT is facing and get the Plesk autoinstaller up and running for you.
Additionally, I've worked extensively in VPS, VDS, VPN, VPC, subnet setups and have a deep understanding of OVH servers. That means I can also seamlessly utilize your two purchased IPs for respectively assigning the Proxmox and Plesk machine while ensuring their efficient communication. Finally, I own a robust knowledge base of Ubuntu systems so installing the two ubuntu machines would be a breeze for me. Choose my skills and experience to make this project a success!