Check the follwing possible reason.
1.Did not complete APN settings?
For iOS, with the lates OS no manual APN seeeting is required.
For Android, see here
2.APN profile for the other telecommunication carrer is working?
You may need to deactivate or delete the existing profile.
For iOS only 1 APN profile could be installed at the same time, and the APN setting is enabled to all SIMs connection.
Therefore, with this situation our NW connection will be impossible.
For Android, multiple APN settings may be configurable at the same time.
Make sure the APN for JapanSIM(povo.jp) is enabled to the downloaded eSIM profile.
3.Still can't connect ?
See here to check compatible devices condition