You can refer to the following guides to insert a micro SD card into the Reolink Duo series properly.
Applies to: Reolink Duo, Duo PoE, Duo WiFi, Duo 4G, Duo 2, Duo 2 LTE, Duo 2 PoE, Duo 2 WiFi, Duo Floodlight PoE, and Duo Floodlight WiFi
Here is a video for your reference.
For Reolink Duo, Duo 4G, Duo 2, Duo 2 LTE
Step 1. Unscrew the lower part of the semi-dome cover on the camera with a screwdriver, and open the rubber cover on the SD card slot.
Step 2. Insert a micro SD card into the SD card slot. Use your fingernail to push it in until you hear a slight click sound and the SD card will not pop out.
Step 3. Press down the rubber cover firmly and screw back the cover in place.
For Reolink Duo WiFi, Duo PoE, Duo 2 PoE, Duo 2 WiFi, Duo Floodlight PoE, and Duo Floodlight WiFi
Step 1. Use a screwdriver to unscrew the cover on the SD card slot from the camera.
Step 2. Insert the micro SD card in the direction of the micro SD card as shown in the picture below. Please use your fingernail (or use the provided reset needle) to push the micro SD card in until you feel it is locked, and you will hear a click sound and the micro SD card will not pop out if the camera is inserted in properly.
Step 3. Screw back the cover in place for better waterproof performance.