The Yeastar TG device has multiple cellular radios, one for each trunk/SIM.
The IMEI number is unique number identifying the cellular radio. Your carrier may request the IMEI number of the device. You will need to give them the IMEI number of the trunk using the carrier’s SIM.
From the TG device Web UI:
Method 1: Hover your mouse over the Trunk Name. A pop-up will display the IMEI number
Method 2: Click on the trunk, then the Advanced Settings tab. The IMEI number is displayed.