IPsec Site-to-Site VPN Setup

I need a reliable IPsec site-to-site tunnel between UDM SE and Zyxel USG Flex 50 configured between two locations so that both networks behave as if they’re on the same secure LAN. The goal is straightforward: traffic must flow seamlessly and be fully encrypted end-to-end, with no reliance on third-party cloud gateways. I want to be able to access the NVRs between the two locations.

Here is what I expect:
• A working IPsec Phase 1 / Phase 2 configuration on the edge devices at each office (vendor-agnostic for now; I will share current router/firewall models once we start).
• Routing that allows each subnet to see the other without overlap or asymmetry.
• Clear, step-by-step documentation of every setting you touch so I can replicate it later or restore it after a firmware upgrade.
• A brief live hand-off session to verify connectivity and answer any final questions.

If additional ports, NAT rules, or dynamic DNS adjustments are required, please outline them in advance so I can coordinate with our ISP. I am ready to provide public IPs, subnet details, and remote access to each device as soon as we agree on the approach.