Working for an ISP, have you noticed anything in particular at home that could affect your performance? It's possible for exemple at 100 mb/s if someone download, another member use netflix and you play PSO2 that it will cause major slow-down or disconnection (since you modem will try to push more than it can handle).
Plus, the ISP should ALWAYS provide your plan. if you're supposed to have 100mb/s and they do not provide it whatsoever, you can always call them and they will have to fix it no matter what or at least give you a credit.
In any case, I do have 100mb/s and 25 mb/s upload and I can garantee you that I never crashed a single time on my side. I suggest you call your ISP as they can check if anything at home would use internet and download something. (psst, if you have an Xbox one SHUT IT DOWN this console is patching like 24/7 and my client always have slow down because of this console -_- if you are playing on your Xbox one however, you might want to limit your download speed or something if possible)
Anyways, Hope that helps or give you some idea on how to work around your issue in order to solve it but you should DEFINITELY NOT have any problem playing PSO2 if there is NOTHING ELSE using your internet connection.