Aye same for me. If I force to 5G preferred I get nothing. But I believe it is using 5G NSA (i.e over 4G) because of the colour of the led on the unit. I don't have the paperwork to hand but I'm sure it's blue if "5G" and green if not. Or something like that.
You could try manually picking the bands the unit is allowed to use as well. It could be (not sure on your SIM provider) that it's using a lower frequency band for stability. So mine usually prefers band 3 with EE because of the distance from the tower but with the antenna on the roof I can use band 7 (turn off band 3) and get a signal from the tower about 4KM away. Obviously, band 7 is higher frequency = more throughput. I do get more lag and jitter though. But band 3 isn't slow, I easily get 250Mbps.
So perhaps that may be a reason for the speed difference? Or perhaps placement?
Future FW updates may help, you can check here -
https://www.zyxel.com/support/download_landing.shtml and pop NR5101 in the search box (
https://www.zyxel.com/support/DownloadLandingSR.shtml?c=gb&l=en&kbid=M-03419&md=NR5101). Nothing available yet.
EDIT: Just checked the Cellular info from the Router:
- Access Technology
- 5G NR-NSA EN-DC
That's with the preference left on "Auto".
EDIT2: Yep and if you force 5G - you get nothing. And if you force 4G you just get, 4G (see below). So it defo is best on "Auto" as that does make it use 5G.