Sorry, missed the instructions.
It turns out that it was a browser extension that was causing the problems. Not sure why, as everything worked fine up until recently. Regardless, it is nice to have this resolved.
@leocg I did try, that didn't work on the moment, and after 2 days changing absolutely nothing so maybe an update happened in the background or I don't know, it just works, I seriously have no idea I didn't try anytbhing specific but it works now.
@leocg Thank you so much for replying, I just installed regular Opera, and it looks the same, I can't enable subtitles.
I found the answer in Reddit apparently I'm not the only one with this issue.
If anyone else comes across this problem, I solved by installing User-Agent Switcher and switching agents to Firefox-Windows in HBO.