For iPhone
Go to Settings > Cellular > Cellular Data Options or Settings > Mobile Data > Mobile Data Options.
If you see the 5G option, then your device support 5G.
If you only see 3G or 4G/LTE options, then it means your device does NOT support 5G.
For Android
Each device model may have different Settings menu, but you can try go to Settings > Connections > Mobile Networks > Network Mode or try search Network on your device. If it does not have 5G option, then it means your device does not support 5G.
※This 5G service requires your phone to support 5G frequency signal (Band n28, n78, n79, n257). If your phone does not support these, then you may not be able to receive any service. Please check the compatibility before changing.
If you would like to use 5G network option in your SIM card, please read here for further details.