Route outgoing traffic through different ip

Hello to all, I'm trying to route outgoing traffic through different public IP addresses.

This is the current setup

At my FortiGate 200D (v6.0.3) the WAN1 port configure with a manual IP address that I got from my ISP, for example, the IP is - 31.150.10.116/255.255.255.240. 

This WAN1 is connected to a Cisco 1941 router with the static IP 31.150.10.113/255.255.255.240 

and at the FortiGate the static route set to:

Destination : 0.0.0.0/0.0.0.0 -> Gateway: 31.150.10.115

At the current setup the trafice route through the Public IP - 31.150.10.16

Now, I like to remove the cisco route and do all the routing through the FortiGate appliance.

The issue is that when I set the WAN1 Port with the ISP IP 31.154.12.50 + static route to 31.154.12.49 I'm losing the former IP address (31.150.10.16)

Anyone has any idea how to do it?

Thanks/