OK. That looks bad. I took a ZTE Blade V2 right up to my rural mobile tower and it performed badly. I put it down the the tower transmission being so strong that it caused distortion in receiving. The Blade does 57 from 8km, and I think it was 27 standing next to the tower. The Huawei Y3ii does 67 at 8km.
Another factor that can influence performance is wifi. It looks like you are doing the iPhone test directly on the phone. This really speeds things up. For example I have an Acer laptop that does about 32 by wifi and 64-100 by usb from a Netgear 790s.
Some wifi doesn't work well. How can you eliminate that for testing?
Eliminating wifi - the only device connected to wifi at the time was the iPhone.
Even then you might expect +-1 or 2 Mbps but not this kind of difference.
I throw another option in, the modems both have Optus branded firmware, the iPhone doesn’t.
I can eliminate wifi.
use laptop and tether to iPhone or modem via USB but I’d be surprised if the results are different
As I said, wifi to my laptop HALVES the performance. My neighbour and many others can't even get apple products to work through ZTE/Huawei modems.
I was doing a download in my car, line of sight to the tower and could only get about 3.5megabytes. Then I remembered that I had to use USB. It can catch you off guard.
I've never seen modems behave in such a weird way. How many towers have you tried? Maybe your tower wont support aggregation. If that doesnt work, then raise it on Whirlpool, or try to get the attention of Paddylee, here.
It occurred to me that you should check what tower number the various equipment is using. Could be the iPhone is locking in on a more distant uncongested tower. You would need to load a special cell app to get the number of the tower. The modems say what the cell ID is, in their information sections. The aggregating modems might be fallaciously locking to a congested tower. The bands that they aggregate have to come from the same tower, so they can't pick and choose. Also try to work out what band the iPhone is using.