Включите ip forwarding у клиента и у сервера:https://www.ducea.com/2006/08/01/how-to-enable-ip-forwarding.../
На сервере поставьте чтобы шел в 192.168.20.0/24 через подключенный IP клиента, скажем 192.168.1.71:
ip route add 192.168.20.0/24 via 192.168.1.71
И попробуйте как-то так:
Server:
[Interface]
Address = 192.168.1.70
PrivateKey = <server's privatekey>
ListenPort = 51820
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE
[Peer]
PublicKey = <client's publickey>
AllowedIPs = 192.168.1.71/32, 192.168.20.0/24
Client:
[Interface]
Address = 192.168.1.71
PrivateKey = <client's privatekey>
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE
[Peer]
PublicKey = <server's publickey>
Endpoint = <server's ip>:51820
AllowedIPs = 192.168.1.0/24
PersistentKeepalive = 25