At the URL opera://settings/onStartup, make sure you have "Retain tabs from previous session" ("Continue where you left off" in Chrome) selected and make sure you close Opera view "Menu -> Exit" without closing any windows.
Then, when you start Opera, I would think that as long as all monitors are connected and turned on, and in the same configuration as before, when you start Opera, all of the windows should open up on the monitors they were on before.
If that doesn't happen and it works fine in other Chromium-based browsers, I'd consider it a bug in Opera.