Who is responsible for investigating a connection problem: Centrii, my ISP, or the O&M provider?

It depends on where the fault is. Centrii owns the monitored connection and the Centrii equipment on site. The physical circuit belongs to your internet provider, and the SCADA / turbine data path (for example a turbine vendor's own link) belongs to that vendor or your O&M provider. The ticket is the single place where all parties coordinate, so raising one early is the fastest route to resolution even when the root cause turns out to be third party.