NAT Port forwarding to VPN Client

Dear all,

Public IP in Office: 213.123.123.124

Office LAN:

192.168.9.0/255.255.255.0

We want to setup a SSL VPN from a remote site through 4G Network.

VPN IP range:

192.168.100.0/255.255.255.0

How can we setup NAT port forwarding to VPN IP? 

For example, 213.123.123.124 TCP Port 1400 to 192.168.100.20 TCP Port 1400

Ivan