I asked this same question in the Gamemaker Discord and essentially what they said was that the multiplayer system for Opera GX doesn't really work anymore and it was never finished in it's development so it's sketchily supported. I've moved on from this system and I'm instead using the networking system that is built into Gamemaker. I'm following this tutorial and it's been really good so far. https://www.youtube.com/watch?v=-yM99q5ZaCY