Found the problem.. According to GSMArena it looks like the V3xx (which I think is that type of phone) only does 850/1900 3G. The issue would be that T-Mobile is in the process of repurposing 1900 3G spectrum as B2 LTE.

I'm sure you can find a newer flip phone but yes.. that phone will likely stop working even if - in your area - it works right now. I'd be willing to bet the seller on eBay knew that too. 

 This phone would work for you. 

https://www.amazon.com/Nokia-2780-Unlocked-Verizon-T-Mobile/dp/B0BLDZDN9R/ref=asc_df_B0BLDZDN9R?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80814222249878&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584413757970024&th=1 

 But for overall durability and price these are excellent. 

https://www.amazon.com/CAT-FLIP-BLACK-T-MOBILE-LOCKED/dp/B0BZQKYKQF/ 

 Actually buy the unlocked CAT flip. Supports a broader range of bands and carriers. 

https://www.amazon.com/Cat-S22-Flip-Touchscreen-Resistant/dp/B0BTTQGVVW/