Home Internet - 4G LTE vs 5G

I added a couple dedicated cooling fans (one on top and one on bottom) to see if it helps the stability of my connection.  After doing this and moving the modem back up on to a table from the floor, my primary signal is on B66 at 6/6 bars.  My secondary signal is disconnected.  I just did a speed test and got 77 Mbps down and 46 Mbps up.  These speeds are consistent with what I've gotten in the past which makes me think my secondary signal has never been used because it's been too weak.  According to cellmapper the tower I am closest to is only 4G.  And if I go back to the T-Mobile page to check if I'm eligible for the service, it says I'm not.  I wonder if I got it by mistake.  Hopefully my tower will be upgraded at some point in the future and I'll have a nice 5G connection.