Bridge PPPoS with ethernet

I have a PC on a test bench with 2 interfaces, enp2s0 and enp3s0. The IPs are public and obtained with DHCP, based on the interfaces MACs. There is a second computer on my bench, a Raspberry Pi Compute Module 3, which I want to expose on the internet. The RPi and bench sever are connect together through PPP, over a serial connection. On both side the ppp interface is called ppp0. How can I bridge ppp0 and enp3s0, so that the RPi CM3+ accessible from the public IP? I'm lost among linux's many bridge and tunnel options... My organization doesn't allow me to do routing or use switches. Any solution that minimize the chance of me wrecking the rest of the network would be awesome.