5g disappeared after bootloader unlock

[DAR-FAISAL]

My device (Samsung A15 5G, One UI 8.0, Android 16) originally had OEM Unlocking missing, but I needed to unlock the bootloader. So I downgraded the device to Android 15 (One UI 7.0).

After I unlocked the bootloader, my 5G stopped working.
When I select 5G in the settings, the network gets crossed out and both network and internet stop working.

But when I select 4G (LTE), the internet works normally — only 4G (LTE).

When I select 5G, both the network and internet completely stop.