Using AT&T phone number for texting in Europe/ Serbia

I would like to share my conversation with an agent. It may "help" the other users.

AT&T: Hello! How may I help you today?
Me: Hi
AT&T: Our agents are currently assisting other customers, your approximate wait time is 3 minute(s).
James: Hello! My name is James. I hope you are having a great day! How may I help you today?
Me: My name is Dragan
Me: Just posted a question on a forum
Me: Hi,
I am using AT&T service while I'm in US. Now, I went back to Europe (Serbia) and unfortunately I can't use it. I can see all available networks, but cannot connect to any. I need it to receive sms, don't need it for data. What settings are wrong? Also, I have to mention that I just refilled my account and it's valid for a month.
Can you please help?
Thanks
Me: hopefully someone can assist
Me: I need it to get an sms in order to re activate my Viber acctoun
Me: account
James: I can help you out. Are you currently in the US?
Me: no
Me: in Serbia at the moment
Me: on my vacation
James: Are you using your AT&T phone?
Me: I have my personal unlocked phone
Me: I can use any simcard anywhere in the world
James: Are you using a Serbian SIM card?
Me: I'm using my old Serbian number and also i would like to receive an sms on my AT&T number
James: Can I have your AT&T phone number?
Me: So, I have 2 simcards at the moment- Vip Serbia and AT&T
Me: sure
Me: +1808785XXXX
Me: When I insert the ATT simcard I can see 3 available networks, but unfortunateley can't connect to any
James: Alright, so the SIM you have, since it's a GoPhone SIM, won't work in Serbia.
Me: So what options do I have?
Me: Because a friend of mine can use his T-mobile here
Me: Why I can't use my ATT?
James: It's because it's a GoPhone SIM, you could use a Postpaid SIM there, but not a GoPhone SIM.
Me: When I've decided to use ATT network, I went to the store on Hawaii and the agent suggested me to get this one. Since I'm not US citizen, I couldn't get postpaid card. I'm just working for one of the major cruise lines companies.
Me: I have to reset my phone now
Me: And I can't get my viber code
Me: Untill I have service on THIS sim
James: I totally understand, unfortunately, a GoPhone solution wouldn't work in your current situation.
Me: How and when can I do taht?
James: Unfortunately, that GoPhone SIM won't work outside the U.S.
Me: So, If I understood well-
Me: There is no chance for me to receive an sms untill I come back to US?
Me: Correct?
James: That's correct.
James: Unless we can get that Viber code sent to your Serbian number.
Me: I can have it on my Serbian number, but that's compliteley different account
Me: I want to use my current account
Me: And can't untill I receive an sms on my US number
James: Right, I understand that, and I do apologize that you were misled, but unfortunately, that SIM won't be able to work overseas.
Me: That su*ks
James: I definitely do understand.
Me: Since I'm somebody who is living in several countries
Me: Someone could tell me before I've got this number
Me: And no solution for me untill I go back to US;
Me: very dissapointing
James: It's possible that the rep didn't totally understand the situation, and provided the GoPhone option without knowing you'd be using it in different countries.
Me: Yeah, right 😉
James: Is there anything else I can help you with today>
Me: No, I don't think so.
James: Thank you for contacting AT&T and you have a great day!
Me: U2

I'm not sure I understand where your going with the map.  That is postpaid roaming coverage.

Postpaid accounts can add a roaming plan and use their ATT phone number.   You cannot.

Gophone plans are limited as I posted and the chat confirms.

If you travel often, you should consider moving your number to Viber or google voice, so you can collect calls and messages no matter what SIM card you use.

@lizdance40 

Thank you very much for trying to help.
NOW I know what is the best solution, but the problem is when I've decided to get an US number nobody told me that I can't use it somewhere else than US, Canada and Mexico. The other operators have an option to "switch on the roaming" giving a simple call to the operater before the trip. ATT doesn't (at least with gophone)! Since I'm not US citizen (don't have the social security number), I couldn't get the postpaid account.
And now the problem is that Viber can't use the other (backup) phone number or email or whatever to resend you a code. I can use it with my other number but isn't the same account and I have to invite ALL my contacts to join me again without saving previous conversations.
That s**ks 😞

Yep.  That sums up the problem well.

Sorry you didn't know the limits of prepaid plans.  All prepaid and most small US carriers have no international options.