I ran into this issue also because I want to use both my on board (enp1s0) and pcie (ens6) ethernet adapters.
This is what worked for me:
1. Use omv-firstaid to setup static ip on enp1s0 - 192.168.1.x
2. This created /etc/netplan/20-openmediavault-enp1s0.yaml
3. Create new yaml file for ens6 cp /etc/netplan/20-openmediavault-enp1s0.yaml to /etc/netplan/30-openmediavault-ens6.yaml
4. Edit /etc/netplan/30-openmediavault-ens6.yaml for it's ip address
5. netplan generate
6. netplan apply
7. Reboot for good measure
This seems to function for me on my OMV5. Only the 1st adapter shows up in System / Network but both appear in Diagnostics (Network Interfaces). Also only the first (enp1s0) appears in the Diagnostics / System Information / Performance Statistics / Network Interfaces graphs.
I'd really like to know how to fix those annoyances, but at least both adapters are function