I've been running OPnsense on a desktop box with dual 10GB NIC's for a while now and have purchased a UDM Pro Special Editon with UniFi switches, WAP's and security cameras and am having difficulties with the intial setup of having the UDM Pro behind the OpnSense box.
I have viewed and tried the information contained in YouTube videos by Lawrence Systems https://youtu.be/Omm2pQUJO0o?si=9q7MLu4Hwg4Qm3cm and Mactelecom Networks https://youtu.be/Ewq8TvMY6Jo?si=kp-CubCXB3ntBVs4 as well as surfing the net but am unable to get the UDM Pro to get an IP ins the same subnet as the OpnSense box as outlined in the tutorials. Does anyone have any advice with achieving this?
The UDM Pro SE has just two types of ports: Explicit WAN (and optional WAN2) ports and LAN ports.
Then you have networks. You can define the UDM Pro's own address by setting the "Gateway IP/Subnet" in the Default network. Normally, this is 192.168.x.1/24, but since your OpnSense probably uses that, you should use any other IP, like 192.168.x.2/24.
I think (never done it before) that this is the main reason why you must also use the WAN in order to have UDM software updates: This setting also sets the default gateway in absence of a true WAN connection (or the UDM can only use the WAN connection as default gateway, which is what Tom describes).
Also, there is a more detailed video on that by Mactelecom: https://www.youtube.com/watch?v=Va4r_1xSwAY
You may want to consider returning your UDM as it duplicates the functionality of OPNsense (in a worse way, in my opinion). Get a CloudKey instead and possibly an additional NVR if you have a larger number of cameras that the CloudKey can support.
If you want to keep the UDM, the UDM behind pfsense video by mactelecom is great.