Your first post says you can't connect to wifi, and the second that you can't access the internet. They're separate problems. How are you checking the connection status: ethernet/wired connection? That should, theoretically, solve itself if you leave the box turned on for an hour or so as UPC need to switch your internet from whatever you had before to the connect box. It could also be because you've not connected the coax to the correct socket in your wall socket point.
As for wifi not working ... change the password again, and use the same SSID for both the 5G and 2.4G networks, and try again.