I really wish I could help you in your case. I tried looking back into my text messages, but I didn't find the verification code that I know I must have used to activate my phone app. I wonder if installing the app on a different phone would help, or perhaps using a different internet browser. The link I sent you was to the Google Play version of the app, are you using an Apple phone? I know there's a version of it on the Apple store too. Perhaps you could take a screenshot of the page on the Tixplus website that shows you have proof you bought your tickets, print it out, and show it to the ticket counter as a last resort? Other relevant screenshots may be useful as well. Generally speaking, Japanese people always try to help.
Edit: I actually contacted support regarding a payment issue, and they did get back to me a day later. Maybe the best thing to do is to wait. The website says, "Responses to some inquiries may take anywhere from a few days to approximately one week." It also says that customer support inquiries may only be handled on "weekdays 11:00~12:00 / 13:30~18:00 (Japan local time; excludes weekends and holidays)," so you might want to check Japan's local time when making an inquiry if you haven't already.