If you are having issues connecting to Dokkan [JP or Global] use mobile data instead of wifi.

I have already checked both on mobile data and it's working fine for me. I'm in The US, and in the south, if that helps.

Try using your mobile data until you get into the game. Then turn the wifi back on. If that doesn't work, Consider putting in an inquiry about the wifi issues. If there's enough of them, they'll investigate if/what is the issue.

https://bnfaq.channel.or.jp/inquiry/1624

EDIT: It could be, after checking other posts/comments, that it is an issue for people who are in the UK, or Europe in general. I haven't seen people in the states having the issue, on WiFi or mobile data. So it could be an issue with European Wifi/mobile data connections and their ability to connect to the Dokkan server. Check with your ISPs/Mobile Data providers and see if they are having issues with services. Otherwise, put in an inquiry to Akatsuki to the best of your abilities.