CDO will discover a site-to-site VPN tunnel when one of the peers is onboarded. If the the second peer is not managed by CDO, you can filter the list of VPN tunnels to find the unmanaged device and onboard it:
- Click the VPN tab to open the VPN page.
- Select Table View.
- In the Filter panel, in the Onboarded filter, check Unmanaged.
- Select the unmanaged device from the results.
- In the Peers pane on the right, click Onboard Device. Follow the instructions for Onboard a Firepower Threat Defense Device Using Username, Password, and IP Address.