Having trouble contacting Rakuten Mobile

Last August, I left Japan after finishing my student exchange program. While I was there, I used Rakuten Mobile data. Before leaving, I went to their office in Nagoya to discuss cancelling my internet plan. I also confirmed whether I had any outstanding bills, as I had always paid them on time at FamilyMart. They assured me everything was handled and that I could delete my Rakuten Mobile account since I wouldn't use it anymore. I followed their instructions and left Japan without any issues.

However, in September of last year, I received an email stating that I had an unpaid bill for August (the billing month of September). I'm unsure if this bill is valid, as I didn't use their service in August. Nevertheless, I'm willing to pay it since it's a small amount, and I don't want any problems. Unfortunately, I've been unable to reach them or make a payment for months.

Here's what I've tried:

1. I asked a friend to check my dorm for a postcard payment slip and offered to pay them back via bank transfer (my friend couldn't find any slip).
2. I called their overseas contact number multiple times, but they couldn't understand my concerns as they only speak Japanese (which is completely understandable).
3. I tried calling them again with a friend's help, but they wouldn't proceed because the caller wasn't me.
4. I can't pay by credit card since I deleted my Rakuten Mobile account before leaving the country, which means I no longer have access to it.

I'm out of options. I genuinely want to pay the bill, but it's incredibly difficult to reach them. I was confident I had no outstanding bills and even consulted with an employee before leaving Japan in August. This is concerning because I plan to return to Japan for studies or even to live there for a while. Thank you so much. Any help or suggestions would be greatly appreciated.

I would try (3) again. This time do a three-way call so that you are there and can approve the transaction. Or do (3) and ask your friend to pretend to be you.
Or (5) -- don't worry about it. Pay it when you return to Japan if you try to use Rakuten again and it's still a problem.