public ip redirect for an application

RouterOS General
1

Hello everyone

here is my problem:
i have a public link from an app that i need to redirect to on another ip.

let me explain :
I have 2 WAN interfaces
1 XDSL WAN
1 WAN LTE

when the XDSL connection is working the source ip must be changed:
example :
source ip: 185.212.80.X to 100.100.40.X

when the XDSL connection goes down, the traffic goes through LTE, but the destination ip of my application, must be equal to the source:
example :
source ip: 185.212.80.X vers 185.212.80.X (desination unchanged)

I don’t know if my request is clear

Thank you for your help