VPN between Sophos UTM9 and OpenWrt 19.07

Hello all,
unfortunately I was not successful in my search, so I'll try here.
I need help setting up a site-to-site VPN between openWRT 19.07.7 and a Sophos UTM9, preferably via openvpn.
Does anyone here have some tips on how to set it up? Sophos is known but how do I get the apc file suitable for openwrt and how does the setup generally work on the device with openwrt?

thanks to you

Route into a Open-VPN-Client Subnet - #2 by vgaetera