The MiVoice Business does not support phones connected via NAT except with the use of a Mitel Border Gateway, either stand alone or as part of a MiCollab Server...
As NEPhoneGuy stated, I think you are confused... The options you found are for a different system than the MiVoice Business system you have, remember that Mitel has multiple different platforms that share common devices (phones), but that doesn't mean that all systems have the same capabilities.
Is it possible to run a 5220 using Minet through a NAT with a MiVoice Business system? No, not without a Mitel Border Gateway server