Why can’t my primary card send text messages to my secondary card?

Because it is not set to use the secondary card to send text messages, the solution is as follows:

1. First, open the settings on the home page, as shown in the figure below.

2. Then find "Cellular Network" in the settings and click on it.

3. Then in the cellular network, open the settings of the two cards and click on the two cards respectively.

4. Now click on the data traffic card, then open the card and click "Enable this number". As shown in the picture below:

5. After enabling the two cards, enter the SMS page. Find where the arrow points and click on the green box.

6. Click the green box and find that you can change the card used. Selecting the secondary card here means using the secondary card to send text messages.