Devices or units? This page is about devices, the tracking hardware. A device carries up to two SIM cards in its SIM Number 1 and SIM Number 2 fields. You fit a SIM on the device, not in the SIM list.
In the side menu, select Devices.
The Devices list opens.
Select Edit on the device's row.
The General tab opens.
Select SIM Number 1.
A list of your SIMs opens under the field. Each row shows the MSISDN and the operator.

Optional: type a few digits to narrow the list.
Choose a SIM.
SIM Number 1 and SIM Operator 1 fill in.
Optional: repeat steps 3 to 5 in SIM Number 2 for a second SIM.
Select Save.
The app shows "Device updated successfully". The SIM is now fitted: it leaves the Available count of the SIM list.
The list also offers SIMs that another device already uses. Saving one of them fails with "SIM number already exist".
You can correct the record later in the SIM form. See SIM form.
A SIM sits on one device at a time, so first take it out of the old one.