Because its wireless broadband, there's a reason the world's internet backbone runs on fibre.
with wireless broadband, a million things can change speeds from second to second.
And ISPs make you run speeds test on a cable.
If the tests are repeatedly giving the same results, and I mean retesting also at different times of the day then surely there must be something wrong.
Look at the huge variability in ping times in all the tests. You are not getting reliable service. So the average speeds are likely also highly variable over time and not a factor of the SIM type.