iPhone 12 showing LTE Insead of 5G

E, 2G, 3G, 4G, LTE, VoLTE, 5G all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you. About the LTE options on your iPhone - Apple Support (IN)

See the Pic below and explanations thereof

The pic below is self-explanatory

However iPhone model specific feature is listed here --> Identify your iPhone model - Apple Support . and tap on "See the Technical Specs for Model XXXXXX" under each model and Scroll down to "Cellular and Wireless" section.

Know it here --> All about G, E, 3G, H, H+, 4G, VoLTE, 5GE and 5G symbols ...

Keep the iPhone updated to the latest iOS always and never Jailbreak. That's it.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed them on your device or/and have Jailbroken. 

It (Hacking) also depends on how careful you are in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password, etc with your friends and family members.

Be judicious when sharing the device's sensitive and valuable information with friends and family members.

If you don't reckon what is said above is true then you have two options...

1. Report the hacking incident that you are facing to the local law enforcement authorities and follow it up
2. Believe it, it can't be hacked. There are people who still believe that the earth is flat and also some believe no one has ever landed on the moon. What can anyone do about that? This is a free world you can believe in anything, the choice is yours.