Horrible Reception and Slow Data Speeds

I recently switched to AT&T from T-Mobile 4 lines. I have had nothing but issues with slow data speeds and reception. I go to places that I’ve usually always go to and never had problems with T-Mobile. In the city of Downey, CA and in the city of South Gate, CA. At my house I don’t have good reception with maybe 1/2 bars but it still says I have 5G. I walk down 2 houses and it says I have 5G+? How is that possible? I went to a AT&T corporate store because I was by there anyway and asked about the bad service I’m getting in these areas. The guy tells me that some phone companies have priority and somewhat ownership of those areas and that’s why the service is spotty. Can someone tell me if that’s true? Also what’s the deal? I honestly thought AT&T had somewhat better service than T-Mobile and Verizon.