Don't rely on simba in japan.
Tried simba in japan in early March, connected at airport and used about 130MB.
Then unable to connect again. I topped up about 30 bucks for roaming usage but in the end unable to use for data.
Airplane mode on and off. Saw that I am connected to a local telco then disconnect me again, as in no reception no bar. So the days roaming only worked for 1 or 2 hours at airport then never again for the rest of my trip.
Check out their Facebook, got other people complain as well.
Bought a local sim in the end.
Msia no issue though.
What's your phone model and which part of japan?
Could be helpful if you can share more details.
I think someone mentioned need to turnon volte.
For my case I suspected was actually my samsung A7 only support 2 out of 6 bands that KDDI use. Swapped sim into xm note 10, support all 6 bands, no problems with reception at kyoto osaka kobe
Initially thought use my xm note 10 for only photo taking to save battery, use Samsung for navigation.
My mum redmi note 9T on senior plan, no problems with reception from airport to ulu shrine hill