I have the same problem: 3G is fine, but there's no 4G at all.
I've been with Virgin Mobile for years, but only 3G. A few months ago, in preparation, I upgraded to a 4G contract. (That was a headache in itself — the new 4G-capable SIM made my Nokia E51 crash… I had to get a new, cheap dumb phone.)
Now I have a Gemini, I can make and take calls fine, but there's no mobile data. The top of the screen shows ‘3G’ and a mostly-filled triangle (next to an empty triangle, which I guess is for the empty eSIM slot — I dunno why it insists on showing that when there's no eSIM set…). I've set ‘Preferred network type’ to 4G, and ‘Virgin 4G’ as the preferred network, but its shows ‘No service’ when trying to access the Internet away from wifi.
Not having any other 4G devices, I can't check whether the problems with Virgin or the Gemini. I initially assumed there was no coverage at my home, but I've tried it at a couple of other locations too, and Virgin's coverage maps shows there's coverage in most of the surrounding area.