Marshmallow update caused data roaming to (mostly) stop workin

[AC Question]

I let my phone self install marshmallow a couple months ago and since then have been unable to access data while roaming most of the time. Usually it gives me the icon that I'm connected to data but various apps say that I'm offline. I don't know what kind of magical stars have to align to sometimes have data roaming, but most of the time all I can do is call and text. I have already factory reset my phone twice and have called my service provider multiple times and Samsung Galaxy once. They say I may have to send my phone in to be serviced. Does anyone have any suggestions? Would installing a previous version of Android fix this? I never had any issues with data roaming before this update. Samsung Galaxy S5 on Sprint.