protect DMAradius manager by mikrotik firewall

Hi..
My Mikrotik V5.7 have 2 NIC ( 1 for wan & 2 for lan )
I have a radius manger V3.9 connected to a Mikrotik with a real IP for booth (connected with wan switch)
I need to protect the radius manager by connecting it to MK with a fake IP (connected with lan switch) & in the same time I want to have a connection with my radius by the internet ( 3G, DSL …etc) to managing it if I was out of my company, with a rule in MK firewall to protect it
also i want to but an ip like to real for the radius manager to hide it from my users.
ex:
if my lan pool ip is 192.168.88.1/24
then the radius ip must be like 162.184.174.163
Any idea for doing that?

Thx.