Device specific problems? read more posts on here, some users that have same phone as i do, but have problems i don't.
As for 5G, the speed depends on range, and obviously on how many users are on it, the closer you are to the antenna the better, in my city the speed on my two 5G phones won't go above 200Mbps download, don't know about the up speed tho.
Well, in my contract it states that i won't be getting more than 300Mbps anyway.
I would run at least dozen of speed test with Ookla website on both phones, if possible at same time, then swap the SIM cards and do the whole thing again.
I am not aware of limitation per SIM and speed, but maybe one of the SIM slots is primary?
It was like that some 5y ago on some devices.
It would be also not bad to install some app to display the current speed, not that some updates are being downloaded and that affects the test results.
Huawei P8, Mate10Pro, P40Pro;