How many days on the Dexcom G5 Transmitter since it first used (Activated)?
What exactly was the alert you received?
If the Transmitter failed in less than 90 days of operation, you likely can get it replaced under warranty from Dexcom. Dex Tech Support is 24x7x365.
EDIT: On your receiver, the “Activated” date is available under the “Settings” menu. In case you don’t remember the exact date off the top of your head. (I never would recall that.)
EDIT2: (lol) If you call Dex Tech Support, it is faster if you have the box for this particular G5 Transmitter in front of you as they will ask for information on the label of the box.