I don't think it matters if you are in stock ROM when flashing the modem.. I would highly recommend to check out first the prerequisites before installing the new ROM to avoid issues and less troubleshooting.
Let me know if you are able to do the WCDMA... if not there is another way of forcing it using a different flashing software. I will dig the old thread and look for it if needed.
really is there another method, can you help me with the other method. can you provide me the link please or the method