IMSI is the numerical code given on IMS registration https://imeicheck.net/blog/what-is-imsi-number-how-to-check-and-lookup-imsi.
Some more clues to the need for IMS in that blog post.
Hopefully this is the IMS of the new card and not the ghost of the SIM card before Factory reset (though I doubt this is likely).
Good to include a restart after changes so that those transfer to SIM.
If you are on the carrier network if you are getting mobile data; try @badslipper