AT&T makes me using other cellunlocker to unlock my device!

Why did you not buy the unlocked version from Samsung? 

So are you saying that this is my fault? Buying an unlocked phone from samsung is totally not related to this issue which ATT must resolve!!!! 

*buying a locked phone

Are you sure your phone is locked to AT&T? Which model number did you purchase?

If you purchase the unlocked USA model it would work on AT&T just like the AT&T model (wifi calling and 5g!) except that it would not boot with AT&T's blue globe, and is not locked to AT&T. 

There is absolutely no reason to purchase a carrier-branded model Samsung, Google pixel, when both of those manufacturers have full service on a t and t, as you can see by at&t chart. 

Service capabilities of unlocked phones that will work on AT&T now that 3-g is going away

I have been using the prepaid ATT sim card for years.

There’s your problem. By putting a prepaid SIM card into the phone you have imposed the prepaid 6 month requirement to unlock. That requirement is intended for prepaid phones sold at a discount. You can probably get ATT to unlock by filing an FCC complaint.

Hey there hyojkang, we want to help get your device unlocked.

Let’s meet in a Direct Message to get your device unlocked. Please check your Direct Message Inbox (it’s the chat icon next to the bell icon in the upper right corner of the Forums).

In the meantime please review that the unlock requirements are met. 

• Is an AT&T phone or device and isn’t active on another AT&T account.
• Was paid in full when you bought it (not on an installment plan). You won’t have to wait 60 days to unlock it.
• Has a paid-off installment plan.
  • Paid through your monthly bill: Submit your unlock request at the start of your next bill period.
  • Paid off early: The wireless number assigned to the device must be active for at least 60 days. Wait 48 hours after you make your final payment to submit your unlock request. Learn how pay off your installment plan.
• Doesn’t have a past-due account balance. Make a payment to bring your account current. It will take 24 hours for your payment to post. 
• Hasn’t been reported as lost, stolen, or involved with fraud.
• Is an AT&T PREPAID phone that has had paid service for at least six months.

We look forward to meeting you in a DM to help get your device unlocked. 

Thank you for contacting AT&T Community Forums. 

Lorie, AT&T Community Specialist