We are sorry that our FAQ was not able to solve the issue you are facing.
Therefore, we would like to ask you to send us an email with the information required, so that we can speed up the trouble shooting for you.
Our email address : contact@sakuramobile.jp

Required information
1. Screenshot of the page that shows all your current SIMs.
iPhone : "Settings" => "Cellular"
Android : "Settings" => "Connection" or "Network & Internet" => "SIM card manager" or "SIMs"
※ If you can see an eSIM with a Japanese number "+81", then you may be able to fix it by simply doing 2 things 
・Please do the APN setting. 
・Please activate the eSIM.

2. The APN setting page.
iPhone : "Settings" => "General" => "VPN & Device Management"
Android : "Settings" => "Connection" or "Network & Internet" => "SIM card manager" or "SIMs" => Click the SIM with the Japanese number "+81" => "Access Point Name"
※ We would like to check both, if you have entered the correct value, as well as saved and selected the profile.

3. Check if your phone is unlocked (SIM free) or not.
iPhone : "Settings" => "General" => "About" => Carrier Lock section (please take a screenshot for us)
Android : You will have to contact the shop where you bought it from.