We've received user reports and confirmed with T-Mobile's services team that T-Mobile will no longer support the Yeastar device.
Customers have reported intermittent success and failures, so it appears that this is in a transition period.
We do not recommend the Yeastar modem for TopView if you must use T-Mobile.
Alternatives
For voice calls, there's the QikCell RCVX3 Router by tickioT. This device can provide a SIP extension for TopView voice notifications, contact Exele support or tickioT support for a SIP extension.
We are working with T-Mobile to see if they have an offering for texting similar to the Yeastar.
Consider switching carriers to keep the Yeastar.
One customer reported the following from T-Mobile support: "I was able to get the line set up on the Simple Choice Mobile Internet - GVUNLMI Rate Plan! Which also needed a SIM type change from MI-ISP to MI-MI to make it eligible for that rate plan". After this change the SIM worked in the Yeastar device.