Quick one that came up today. Looking on the web for the search term above only found mention that the RRAS server may not have been set to use a static pool of addresses and that is why, in AD Users and computers on the dial in tab, this was grayed out as an option.
The thing was, I was using a static pool.
The answer is, in my case, that this was a Windows 2000 Small Business Server that was still running in Mixed mode. Changing the domain to Native mode allowed me to enter the static IP for a RRAS Dial in client.
The second part of the problem was more confusing. On adding an IP to the accounts dial in properties, and clicking apply, I get the error “member profile changes were not saved because member not found” message. Very odd… Ahh, a reboot fixed that….