I've learned about a way to bypass the mobile registration part. Well, not me but I found those tweets of a TWICE's fan that explain it. I've learnt from them that you can have a global docomo account. Also my credit card got accepted in the following step. I got a vpn too, they have a 30-day refund policy so if it doesn't work at least I can get this money back. The test video worked so I hope all will be fine. Still, the risk is there but if you want to try - now you know the way
https://twitter.com/Tworrtis/status/1510187284997369856