Some users get from their ISP more than one routable public IP address.
In this case you need to assign these IPs to new wan interfaces and then add a DNAT and SNAT rule in firewall.
In our example we assume that our ISP assigned us 100.64.0.0/29 (or 255.255.255.248) with gateway 100.64.0.1 wan interface already has the first available IP, 100.64.0.2, so we'll add .3, till .6 and they will be assigned to internal hosts on IPs 192.168.1.x

Add a new wan interface in /etc/config/network:

config interface 'wan_3'
        option proto 'static'
        option ifname 'eth1'
        option ipaddr '100.64.0.3'
        option netmask '255.255.255.248'
        option gateway '100.64.0.1'

Add DNAT (for incoming) and SNAT (for outgoing) connections in /etc/config/firewall:

config redirect
        option name 'dnat3'
        option src 'wan'
        option src_dip '100.64.0.3'
        option dest 'lan'
        option dest_ip '192.168.1.3'
        option proto 'all'
        option target 'DNAT'
 
config redirect
        option name 'snat3'
        option src 'lan'
        option src_ip '192.168.1.3'
        option src_dip '100.64.0.3'
        option dest 'wan'
        option proto 'all'
        option target 'SNAT'

Restart network and firewall services.

service network restart
service firewall restart