I think I know what is going on with your case, because I am having the same situation right now.
My first NTSC-J Dreamcast was bought years ago, used to play [JP ver] PSO v.1&2 online at official SEGA server.
Unfortunately, it was confirm dead due to the main board failure few months ago.
Then my friend gave me a used Dreamcast which never got online before (Asia version Dreamcast doesn't come with a Modem), seems like it cannot play [JP ver] PSO online even I can connect to internet through DreamPi or BBA.
It is because [JP ver] PSO v1&2 need to check the isao.net ID & PW which save in Dreamcast (BIOS? I think) before enter online mode.
I found this Q&A
from Japanese website said, there is no way to save isao.net ID into Dreamcast anymore because it is impossible to register or login after isao.net ended their service.
In conclusion, it is impossible to play [JP ver] PSO online again unless your Dreamcast has an iaso.net ID saved.
Correct me if I am wrong.