You can do this by using something like Remote Access Server (RAS) connected to your local Network, and have phone lines and modems connected to the RAS.
Once they dial in to the RAS, the remote dial-in user is part of your local network and can use and standard TCP/IP connetivity software like IBM’s Client Access, Praim’s Net-2000, etc. It will be a true TN5250 TCP/IP connection and eliminates the SDLC connection.
A simple one is the MultiTech RASfinder. Not sure if it is available new but you may be able to find it, or another RAS on the used market.
It will work, and still have the security of dial-up from the outside, but will be a TCP/IP connection to the Power7.