Sorry, I don't have basic knowledge of networks, I'd like to get a help.
Customer is using RB-2011uias-in,
And they added fortigate firewalls above the mikrotik router.

After that they couldn't use port forwarding function, couldn't get the public address.

Could you please tell me what steps they need to get a public address?
Need to open a specific port to pass the firewalls?

If they added Fortinet ABOVE the Mikrotik, they need to get stuff fixed on the Fortinet first.
Without detailed picture it is difficult to guess, things might have changed. (eg. before Fortinet public IP was on RB2011, after "slide-in" of the Fortinet the public-IP moved to the Fortinet etc,etc => all possible scenario's

I dont quite understand.
Why do you have a networking client, when you dont know how to config ????