It’s relatively easy to set up a WireGuard VPN that will connect multiple networks or servers. This will allow you to secure traffic to, from, and across the new network. There’s a good level of support for WireGuard site-to-site connections, though many firewall providers still don’t allow you to use this protocol (Sonicwall and Barracuda are two that do).
It’s significantly easier to link two routers, though, with major firmware like OpenWRT, DD-WRT, pfSense, and Asus all offering WireGuard configuration guides on their respective websites.