I've never been aware of anything besides Procomm that would enable you to manage these switches completely, although you should be able to make some progress with the RMX port - there's just a lot of stuff you can't access that way, such as backup routines, adding users and other stuff on the unix side of things.
I have also never been able to find a place to buy procomm in the last zillion years or so. Fortunately I own a copy, and I guard it with my life!! The last company I know that was actively selling it was Symantec. Maybe you could check their website or message their customer service. The Siemens techs must be getting it from somewhere. Otherwise, at risk of angering the forum gods, you might want to seek the wast resources of the internet to see if you can find an .so somewhere - make sure you try to find ProComm for Windows version 3 or higher for best results, but I do have an old laptop with DOS on it and that version of ProComm still works for basic administration - you just can't run some of the advanced maintenance scripts.
My best results with the RMX port and modems came with older MultiTech modems. I hooked the modem up to a laptop at 4800 E71 and programmed in the interface baud rate, then saved it as the default so if the power goes off it comes back that way. Then you can hook it to the RMX port - sometimes a null modem adapter is needed depending on at what point in the serial data path you connect the modem. Once you get that set up you should be able to dial into it and get in. Keep in mind the RMX port is THE most dangerous way to administer your switch if you don't know what you're doing - one little typo can be disastrous!