The way I see it he only needs cyberspace to transform. If a monster could use a portal to exit cyberspace into our realm than so could he.
He just never left through the portal because there was no need. He managed to stop every monster so why enter our world? It would just cause chaos if a giant man suddenly appeared.
So theoretically, if the Ultras needed his help, he could just open a portal near where they were and jump into battle. I imagine he'd need to go back into Cyberspace in order to transform back though.