How to connect a remote server to a local network via VPN?

Question

VPN

Hello.
There is a local office network without public addresses, but with Internet access through someone else's NAT.
The office has a router with DD-WRT firmware.
There are remote servers with external IP addresses and an internal local network.
It is necessary to unite all nodes into one single routed network.
At the moment everything is set up like this:
one of the servers is configured with OpenVPN. In the office, a router with an OpenVPN client initiates a connection to an external server, sets up a VPN tunnel, and connects the networks. Everything works fine, cars see each other. One BUT! The router in the office is weak, data encryption occurs at the software level, respectively, the performance of the VPN tunnel drops three times. Of 2 megabits, 700-800 kilobits are actually squeezed out, which is very sad.
Task : raise VPN and combine networks and squeeze out at least 1.5 megabits between the office and remote servers.
Available Cisco RV180 VPN Router and Zyxel ZyWALL USG 20W
None of them support OpenVPN. But they can work as a gateway.
What needs to be configured on the routers and the Linux server in order to connect networks through Cisco or Zyxel?
As I understand it, the gateway can be configured so that it itself raises the connection to the client, if he is online. But what in this case to lift on a remote server? How can you solve a problem with what you have on hand? It is not possible to install a second router on a remote server.
Or other solutions:
1. Hard-encrypted router with OpenVPN support.
2. Router with hardware encryption and DD-WRT support.
But I have not found these routers yet.

Answer 1

as an option - change the router to an inexpensive PC, raise vpn and nat on it.