Can't return trade-in device at any T-Mobile store - system shows error

I got switched to T-Mobile with three lines and home internet. I upgraded two phones by trading in existing devices. I did it in-store at Transit, Depew. The staff was accommodating. One of the trade-ins was done without any issues. They gave me the new phone and kept the old one. For the second line, they did not have iPhone 14 Max Pro. So, they ordered it and asked me to go in once I received the device as it was shipped to my home. They call it deferred trade-in.

I got the new iPhone 14 Max Pro on Tuesday (Oct 24th). I went to the same store right away. They set up the new phone and tried to trade in the old phone (iPhone 11). However, once they input all the information, the system throws an error. So, they tried doing that on a new tablet and got the same error. 

They told me to go back another day. So I went back the next day (Oct 25th). They still couldn't go past that error.  So, they told me that I could try another store or come back later. They said sometimes it takes 2-3 business days for the system to reset. I called up 611 before going to the store, and the agent said my account was fine and I should be able to return the old device.  

Anyway, I decided to try a different store on Niagara Falls Blvd, Amherst, the next day (Oct 26th). They received the same error and asked me to go back later.

I called 611 again today (Oct 27th), and they said the system should be okay and I should be able to return the phone. So, I went back to the store on Niagara Falls Blvd, Amherst. They received the same error again and asked me to go back later.

Has anyone faced a similar problem? The thing is, I know I can return the old phone through USPS, but T-Mobile mentions Assurant/T-Mobile will not be responsible if the phone is lost or damaged! A few people I know faced significant challenges returning devices through Assurant.

I was planning to upgrade the 3rd line, but the above experience is stopping me from doing that.

Would anyone be able to give me a way forward? I feel if I can't return the old phone at the T-Mobile store, the company should be responsible for shipping. Any suggestions?