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.