change default route for VPN users

I have a linux (currently centos 8 but I can change it to anything) server running as l2tp VPN Server which has two network interfaces as below: NIC1: 192.168.1.4/24 NIC2: 192.168.3.2/24 Default gateway for server is 192.168.1.1 but I want to set 192.168.3.1 as vpn users default gateway. 192.168.3.1 is another router in my network. I currently use libreswan but I am open to change it to anything that does the job! Tried to read ipsec.conf file but couldn't find any related line.