So I haven't used PiVPN before, but I've tinkered with OpenVPN and installed it on a raspberry pi just fine. I've used this for a few years and it's come with me to my new locations (and new internet service providers). A lot of this will depend on getting a connection from the Internet to your Raspberry Pi.
There are two potential problems I see with your setup, and I think the softbank router is the problem.
1. Your home network isn't accessible from the internet.
Internet Service Providers sometimes put you behind a firewall with other users. that means that they do some port forwarding when you use the web, but incoming connections are blocked. Essentially, the IP address you are trying to log into the VPN with is shared with other users.
Solution: Call up your ISP and ask to have a public facing IP address (or be removed from firewall). I've had to do this a couple times. Note that sometimes they will charge you extra if this means setting up a static IP.
2. Your router isn't configured properly (try this first)
When your router gets an incoming connection, it needs to know who to send the connection to. So you need to set up a rule on your router that says something like "any incoming connection on port 1234 send to 192.168.1.2 on port 5678." This is called port forwarding. And you need to make sure that your raspberry pi always has the same IP address. If you are on ethernet, this shouldn't change unless you disconnect, otherwise check out this link if you are on wifi
Solution: implement port fowarding. I just googled "how to port forwarding softbank router" and I got a reddit article saying the router can't do port forwarding :( Bummer dude. I have a netgear router, and the settings for port forwarding are in Advanced > Advanced Setup > Port forwarding.
I hope this helps. And I hope your router will let you do port forwarding!
