There is a not so great work around to install the Web server after the fact that I am immediately aware of.. it is actually easier to do an upgrade from like 11.3 and 11.4 and let it install it at that time if that is an option. Like wise if you want to only install the WebUI on a server but not Prognosis the installer doesn't let you do it.
I have not personally tried this, but you 'may' be able to install it on another server, copy the WebUI folder, over to the other server, export the IIS configuration from the other server, then install the cfg on the other server and it might work at least minimally for your purposes.
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Christopher R Souser - Solution Architect – MSci. PA, CISSP, ITIL.