I use xDrip app on android phone, to start sensor.
Then also start on Tandem X2. Tandem can just pick up same data as xDrip.
xDrip will allow start after day 90 as long as transmitter still has enough battery power. There are also stats and other info.
My current transmitter is on day 3, as shown in my xDrip stats. Also shows how long sensor in use (3.3 days now).
Tandem app simply shows transmitter battery ok, and date/time sensor started.
I use xDrip for most data, and Tandem just for iob. XDrip can send similar data to my fitbit watch.
xDrip is downloaded from internet.
https://xdrip.readthedocs.io/en/latest/install/download/
These instructions are not up to date, but many here can help if you have android phone and want to try it. I think dexcom app has to be disabled.