If your ISP provides a block of IP addresses, you can add them to the "WAN IP Alias" configuration. To use the alias IP, you can either enable "Join NAT IP Pool" for the router to randomly pick one for outgoing connections, or map specific LAN client to the IP by Route Policy. This article introduces how to set up address mapping by Route Policy.
You can add the WAN IP alias from WAN >> Internet Access >> WAN >> Details Page.
For address mapping, go to Routing > Load Balance/Route Policy: create a Route Policy for specific IP address to send from specific WAN IP Address.
With the configuration above, the outgoing traffic sourced from the range of 192.168.1.20 to 192.168.1.30 will use the WAN IP 172.17.1.1
You can add the WAN IP alias by going to WAN >> General Setup, double-click on the WAN profile, enter the available IP address in IP Alias, and Apply.
For address mapping, go to Routing >> Policy Rule to add a new rule as follows:
With the configuration above, the traffic that sent to the Internet sourced from IP address 192.168.1.1 will always use the WAN IP 10.0.0.3.