The GeoPort modems were of very poor quality and were not really modems at all, but DSP devices. That is probably one of the reasons that it doesn't work for you after the upgrade. The work of the modem was actually being shared with the CPU of the computer (which is generally a bad idea) and now that you are using an alternate processor, it doesn't work. I predice that there will never be a workaround.
Now, I notice you are writing from Greece so I have no knowledge of what the requirements are for modems on your local phone system, but purchasing a new 56K external modem would be your best bet for fixing your lack of dial-up connectivity. If you can find versions that are localized for Greece, I recommend Global Village or 3Com brands.