Hi @AliceShanghai,
QGC on mobile isn’t something we’ve ever officially supported (as discussed above), and I’m not sure if the iOS/iPadOS versions of QGC are still being developed (they’re not listed in QGC’s official installation documentation, although in fairness that documentation is also somewhat outdated so it’s hard to know for sure).
From a look at the app store it looks like the last released version is QGC 4.0.10 from July 2020, which should at least in theory be able to control an ROV, although the required setup and connections are not something I’m familiar with.
At a guess they’re hopefully at least similar to the setup for BlueROV Control, which is a paid iPad app by a developer in the community.