I've done some tests with EE and 02 SIMs in the same dongles and the same PC and MAC. Allowing for the usual vagaries of speed with mobile networks, both those networks are giving very roughly the same speeds up and down.
That is not surprising as the limitation will be the dongle LTE category. (Most are Category 4). Depending on where you are in the country you could be receiving only 10 MHz of LTE from O2 on Band 20 (800 MHz) and 40 MHz of LTE from EE on two allocations of Band 3 (1800 MHz). Your dongle could only use one of those at time, your phone can bond these transmissions together (known as CA carrier aggregation)
3 gives faster upload speeds that the others but much lower downloads. It's odd that 3's upload speeds are the same on dongle as on phone, but the downloads are so very much lower.
Often that is an indicator of load. Probably lots of people like yourself taken in my 3's marketing that they are the "best network for data" so using it heavily.
I have looked at the Netgear M1 and similar devices but they are really over the top. There's the battery to be charged and charger to be carried, and you're paying for the WiFi facility. With the USB dongle it's very much plug-and-play and nothing else to be carried.
I completely agree, but if you want something of the performance of a Note 20 or iPhone 12 then you need to pay that sort of money.
Your Note phone is capable of using multiple bands, Band 20, Band 3, Band 7, and multiple transmissions on each band, and joining them together. Speeds of 400 Mbps or faster are possible on EE and close to that on Vodafone in the right areas. O2 can't really reach those speeds in most of the country as they haven't deployed enough of their 2300 MHz spectrum. For 3 to reach those speeds in many cases you will need 5G due to the number of customers using.
So I will stick with the tethering, but I would like to understand the technical reason why 3 has this huge discrepancy,
It is all guess work as the radio signal spreads far and wide. If you want predictability use fixed line broadband, not a mobile network.
Quite easily 3 could sell 100 SIMs to people in your coverage area whom all go home and start streaming 4K video through their phones to the TV. This would impact your download speeds, and nothing you could do about it.
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