Can i just enable 5G for free somehow or do i have to buy a new SIM or package or something? I have AIS, a “Net Marathon” SIM with internet purchased a year at a time for 1700 baht per year and an iPhone 12.

No, 

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Terms and Conditions SIM Net Marathon (10Mbps 100GB) SIM Price at 1,790 Baht (Valid for 12 months) Enjoy internet 4G/3G at speed 10Mbps quota 100GB/month. After the 4G/3G data allowance is reached, customers can use unlimited data at the speed of 128Kbps.

5G is the generation of phones. 

There was 3G, 4G and now 5G. Each new generation of phone has technology upgrades. So if you want the latest and probably the best communication device you would go for a 5G phone.

There is also internet connecting frequencies to a WiFi router of 5 GHz and 2.4 GHz. Loosely referred to as 5G. 

But 5G specification is not the same as 5 GHz ( GHz is Gigga Hertz and Hertz is the old cycles per second and is the frequency of signal proporgation).

So  5G phone will probably be able to receive both 2.4 GHz and 5 GHz signals. (Frequencies attenuate at different levels, so having 2 frequencies makes for better reception).

If your internet provider is only 4G  and your phone is 5G then it will work on a 4G network, but not have all the bells and whistles of a 5G network.

Better to have a 5G phone and use a 5G network.