Upgrade to 600Mhz? what?

So I got the Tmobile home internet and set it up.  My phone connects to it, but why am I getting this message and what do I do about it.  When I called to see if this was available in my area, the Tmobile guy said it was.

Message:

Your device can’t take advantage of Extended Range LTE, our most powerful signal, which reaches 2X as far and provides 4X better coverage in buildings than ever before.  Upgrade to a 600 MHz or 5G device for our best coverage and fastest speeds.