I've also been having this issue doing the same steps I even have made sure to turn on mobile data online play to see if that would work as well as make sure other settings are set to where not a lot of data packets could possibly be overwhelming my phone's system but no luck on getting it to work this issue started after the chaos cubed update got applied I was able to join just fine before that
Update there's been a update but my still unable to join external servers when the app closes I now get the message the app has a bug and to update the app when there's a fix