There are no recently reported SMS outages. Please open a support ticket. Especially for individual cell provider issues, it might be a long time before Garmin notices the problem on its own.
Typically when this occurs, it's because the provider is suppressing SMS messages which contain short links. Garmin uses a short link to the map page which shows your location. As an interim workaround, you can TRY disabling this feature. You can do this in the SMS Message Configuration pane, in the lower right corner of the Account tab at explore.garmin.com. This only affects things on the server. You won't need to sync to make the change effective.
I've already been in touch with Garmin support, a bit of head scratching going on, unfortunately. I have another device running on another MVNO network - it's an iPhone on the Tello network, which uses T-Mobile. That, also, will not allow text messages to get through. On my Pixel, I'm able to force switch MVNO networks (three available.) That, also, has no effect, trying the Sprint and US Cellular networks. I've also disabled "Wifi Calling" on both devices, to ensure traffic is not being routed via VOIP.
I'll try disabling the map link and see what happens.
I did see on a Reddit channel that someone using an InReach Explorer with Google Fi and reports no problems, which is curious.
Probably as a result of the support ticket generated by @phemmert, Garmin is now working on this with T-Mobile. Status now reflects the problem. If you are subscribed to status notifications, you received a notification.
