There’s plenty of posts already about Fampyra/Fampridine. Suggest you perform a search of the forums (‘Primary Progressive’ in addition to this one) to get some kind of feedback.
Just to let you know, I trialled Fampyra a while ago (Last September …I think ?!! ) and had pretty much the same outcome as you’ve experienced. Helped a huge amount for the first few days, but then mobility went downhill severely after that point and is still worse than it was pre-Fampyra. Seems like I react the same way to all meds ?!)