It is very clear now. No need diagram.
Unfortunately you need to set up route in your vpn servers. It could be complicated.
Check this Add static route to other gateway on network (iptables or ebtables professional needed!) - #3 by mrgenie - Network and Wireless Configuration - OpenWrt Forum
and
firewall - How can I define static routes between two subnets in OpenWRT / LEDE? - Server Fault
You can use a third router to join the S2S network and let the cloud to manage the router for you. That is the easiest way.