/diy/ - Do It Yourself

I've got this problem that this sewage hole on the right is clogged, and the bath is being drained through that hole. The bath itself is draining quickly enough, but that sewage hole is draining at the speed of like 1 liter per minute, so while I am having a quick shower, water comes up through that sink.
I know I need to unclog that hole, but it seems I can't open it. I tried twisting it in both sides, lifting it, but it seems just stuck there (keep in mind I did not apply big force while trying that, because I don't want my fingers cut by it)