Site to Site VPN between UniFi USG3 and OPNsense

[tseatah]

Hi,

I've been searching articles on how to do this for the past couple of days now and not had much in the way of success.

Ultimately what I want to do is set up a site-to-site VPN between a UniFi USG3 (running v4.4.57, located in Canada) and a OPNsense server (running 25.7.5, virtualized on Proxmox and located in the UK).

First I looked at OpenVPN as an option, but the config options I have in the USG3 aren't as complex as on the OPNsense side:


I had some slightly better luck with IPsec, but I'm stuck there on the VPN failing to start with the OPNsense logs showing "no private key found for '<my external IP>'"
 
The config for the USG3 on the IPsec side gives these options only:

Does anyone have any idea how I can go about configuring a site-to-site VPN with either of these config options to work with OPNsense? Including, of course, how to config the OPNsense side too.

Thanks.