Product: Credit card or prepaid card

I have been dealing with Citibank for over a decade. I informed them that am a XXXX travelling to the XXXX XXXX most of the year for contracting work. I called to inform them that am going to be back to the States from XX/XX/XXXX XXXX like I used to do especially if am traveling anywhere in the world so they get to accept payments only in the countries I have specified. 
While and during my visit to Virginia, they rejected a payment I have made. I called them to inquire about a payment I have processed and they started asking me a lot of security questions ; I spoke to the Rep. and explained to her that am the owner of the account and am currently in the States for a visit. I told her that I can't understand her accent where I think she got vindictive and put me as a fraud account holder. She blocked my account on XX/XX/XXXX to accept any new charges ( during my visit while I asked requested them to accept charges I have made during my visit ) and after a few weeks, in XX/XX/XXXX they closed it even though I still have an outstanding balance that needs to be settled but I need first to discuss it with them as I think there's a miscalculation from their side but won't be able to do so every time I call and they can't discuss account details of my account since it has been closed. 
Now I can't pay monthly installments when I believe I have been overcharged. During my many calls I have made to dispute their decision, a rep informed me that they could not recognize the international address so they took the decision of closing the account. Its very weird as they always send me renewal to my credit card overseas then how come they cant reach me? I dont think this is correct. I disputed a few charges and they resolved it to my favor until 3rd week of XXXX. How come they took a decision all of a sudden without notifying me first and without sending me any alerts or notices especially that I have never delayed any payments? 
Besides, they could have emailed me an alert or warning notice of account about to be closed or something. I have not received anything from them other than email notifications of the outstanding balance I need to settle on a monthly basis thats all. 

I kept calling them regularly to unblock my account and they kept falsifying facts that they sent me a travel reference number and I kept asking them how did you send it? By email? I didn't receive anything, nor by mail ( send me a tracking number of carrier you send it through ) but they were unable to provide nor by SMS and they kept insisting they did. They kept insisting once I receive the verification code I can call them back with it so they get to unlock my account when finally one of their Reps informed me that my account has been closed since XX/XX/XXXX and they didn't send me any codes of verification of any sort. 
I assured them that I can provide them with any US verification ID to prove am the owner of the account and it's not a fraud account, having been with them for over 10 years and have never missed or delayed a payment and it's all showing on my profile with them but nevertheless, they closed the account without any kind of valid nor reasonable justification nor any warnings or notices that they are going to do so. 
While calling them trying to solve it they kept asking me if I have any assets with the Citi and I think that is the reason ; that I have only a credit card with them that is not linked to any kind of asset, nor any financial products such as : CD, Money market, checking or savings accounts. But I have been dealing with them for years without any kind of guarantee other than having a good credit history ; not only that they have upgraded my credit card for the premier/Thank You Preferred MasterCard client. 
All their pretext is that they can't recognize an international address while I kept my US residence address and valid email that they can reach me at either especially I always get paperless online statements. 
It's an unfair decision that I think has a lot to do with racism and discrimination that one of their reps has started it when I asked her to speak clearly and slowly that I couldn't understand her accent. 
I need my account to be reopened and to be appreciated and valued client ; having been with the Citi for too long plus I don't need anything to affect my credit history as well. 

Please advise of what should be done. 

Kind regards, XXXX XXXX