While I understand your frustration and the importance of the functionality, the issue with AT&T and SMS messages is outside of Garmin's control. This is not the first time that unilateral changes on the AT&T side have adversely affected inReach SMS functionality for AT&T customers. Note that this impacts all inReach devices, not just the Mini.
This is still a problem. It is March 2020 and the problem is not fixed. Garmin can say it is "outside Garmin's control", however Garmin is still advertising that you can text to any cell phone. If this is not true then stop selling the unit or stop advertising that it will work. This is not a toy that it is simply an inconvenience if it doesn't work! It is very much a work tool for many safety related uses. Garmin, get it together!!! We expect more from you
The problem has actually been fixed - and broken again by AT&T - multiple times. Status of the current issue can be found here: https://forums.garmin.com/outdoor-recreation/inreach/f/inreach-general/219217/status-inreach-messages-to-at-t-customers-with-locations-links-not-being-delivered.The current Garmin workaround is to remove the map link from all SMS messages.
If you are willing to permanently live without the in-message link to the online map, you can disable this feature and permanently avoid the ongoing problems with AT&T. The appropriate setting appears in the lower right corner of the Account tab at explore.garmin.com.