You will need to make sure that the phone is 'unlocked'. Most US GSM phone carriers lock their phones so that they only work on their network.
GSM also uses four different frequencies. Which ones are used differ from country to country. There are also differences in how 3G and 4G are implimented. Go to https://www.worldtimezone.com/gsm.html to see what is used in Paraguay, and make sure the phone you send is compatible.