First check whether the mobile phone terminal is set to double 5G terminal. If the 5G\/sa switch is turned on, when the 5G network switch and option are turned on, the terminal has the capability of 5G network, it will automatically receive 5G. If there is no 5G network, it can automatically enter the 4G network.
On the other hand, the main reason is because there is a card that you are using. So the phone card in use will show 5G signal, and the one in use will show 4G signal.
If your mobile phone card does not support double 5g, then only one card shows 5g, and the other card shows 4g signal, and it also depends on whether your Settings are opened in the mobile phone, the double card is 5g signal, if this setting is not opened, or the default may be another card shows 4g signal
The two signal modes of 5G dual-mode 5G, the main card and the secondary card can switch 5G separately, and cannot use 5G at the same time, so when one card is 5G, the other card is 4G.
Dual-card 5g is not supported.