Problems after system update

So yesterday I stopped at another tmo store came across a guy who's been with them 23 years. Looked at my phone asked me where I bought it within minutes showed me my 4g doesn't have the LTE thing so in other words I've been lucky it had been working at all since I never had full 4g…and he convinced me to get a new phone which I'll be doing this afternoon. 

However how come it was working just fine before the update?

And any phones I shouldn't get???