How to make routing with a chain of VPN servers?

Question

VPN

There is a server on Debian, located on the M9 site, clients connect to it via VPN, both with static IP addresses and dynamic ones, this server actually combines offices and mobile devices.
Worth it - StrongSwan and xl2tpd. Offices are connected via IPSEC + l2tp, mobile devices via IPSEC.
We took a VPS over the hill, we wanted traffic between offices to walk through a server at the M9 site in Moscow, and everyone went outside through a foreign VPS.
Debian with StrongSwan and xl2tpd was also installed on the server (the server has a shared IP, and as I understand it, Public IP is transferred to it).
The server on M9 raises IPSEC and l2tp with VPS, after that, if you raise the route towards VPS, then the server ceases to be accessible by its IP.
Actually, the question is how to make the scheme work, how to register routes so that when connected via VPN to the VPS, the server on the M9 remains available for connection for clients, including those with dynamic IPs.

Answer 1

https://habr.com/ru/post/108690/