How to file a complaint about an issue : BOGO Offer + Return Device Refund

Dear Forum Members, 

I was a loyal customer of AT&T from long time. I was facing an issue from March 2019 and had several calls with AT&T customer care around spending 20+ hours of talk about issue for explaining every time to every agent. Final message they are going to fix it, But never resolved. Before approaching Consumer Court, Would like to write a written complaint to AT&T to get it resolved. 

Here is my situation: 

1. Pre-Ordered Samsung S10+ and Samsung S10 in March 2019. One phone is upgrade and another phone is with new line

2. Samsung S10+ was delivered to my current address : No Issues

3. Samsung S10 was delivered to my old address which I left a year back: Problem was started

4. As soon as I received S10+, I did the tracking of S10 and it shown that delivered at my old address 

5. Immediately contacted AT&T and requested to route to my new address, But they mentioned that it is not possible and only option is to return S10 device to get new one. 

6. I have contacted my colleague/friend who is living near to my old address and used return label which was shipped with the device and dropped at UPS store without unpacking S10 device

7. Again contacted AT&T to ship new device, But they mentioned that returned device needs to be reached their warehouse to ship the new device. 

8. Received return received email from AT&T on March 22nd, So I have contacted AT&T to ship the new device. 

9. This time I have requested to ship S10+ as I liked it as per the BOGO offer which I was originally pre-ordered.  Call was forwarded to the another team to handle it as current BOGO only offer to get S10 if you brought S10+. After my discussion they agreed and shipped new S10+ device

10. It was happened last week of March 2019

11. After that I have contacted AT&T to issue refund for the device S10 which was already returned. Agent was promised that it will be credited in 5 to 10 business days. 

12. It was 5+ months now, Still I didn't get refund and had several calls with AT&T agents and always got a response that I am going to get refund in 5 business days. In last call they mentioned that they can't refund and can do account credit. It was mentioned 2 months back, Nothing was credited to my account until this time (10/17/2019)

13. Another issue with the new device which shipped later, AT&T is charging installments and saying that BOGO was not applied because of both devices are not purchased at the same time. 

After all this experience and conversations with AT&T Agents and escalations, I am feeling that to file a complaint at Consumer Court to get it resolved. 

Help me out the best way to resolve this issue.