Why is my Mac's upload speed so slow on Wi-Fi only?

if a VPN solves the problem, it sound much more like a DNS issue than a Wi-Fi broadcast issue.

Wi- Fi Radio Signals from your router fall off as the cube of the distance if you have clear line of sight and no competition from neighbors' Routers or known interference producers like microwave ovens.

All other need to do a more careful analysis to find out what is happening.

Wireless diagnostics:

Hold down the Option key while you click on the Wi-Fi icon on the menubar to open up the tools for investigating and fixing Wi-Fi issues

First is "Open Wireless diagnostics", which opens the wireless diagnostics Assistant, but does not proceed.

There are about eight different things you can do from here, but the top-level is to choose Diagnostics off the Window menu, or simply click continue, to do a quick check for Gross misconfiguration or operating problems. Your Admin password will be required. If any recommendations are shown, you should consider then seriously.

Next is to hold down the Option key and click the Wi-Fi icon as before. The screen that opens shows operating parameters of your network. Screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

That's a very unusual frame size. 1500 is standard for Ethernet V2, there are a couple of slightly smaller common sizes (1480 and 1492 bytes) when things like PPPoE are in play - but usually devices will negotiate that automatically.

Interestingly, 1280 is the minimum frame size required by IPv6 networks...I've never heard of DMSE as a sub-brand - but I do wonder if your ISP is forcing IPv6 and there's something between the router and the Ubiquity Wifi that's negotiating differently from the Ubiquity to Mac connection and dropping the Mac MTU is forcing a lowest common denominator situation that prevents odd packet fragmentation between the Ubiquity's inbound and outbound buffer queues.

I mean, you would expect Ubiquity devices to work together seamlessly...but maybe the ISP provisioning profile isn't optimal.

The suggestion that this is a Mac limitation is not likely to be correct.

Apple supports very large MTU sizes, up to 9,000 bytes.

However, studies done by readers here and documented in older discussions found that a LOT of random equipment did not support MTU Larger than standard 1500. Large frame size implementation was impossible to predict and fairly complex to decide -- when it could send large packets though, and when they were forced to be fragmented by the receiving equipment.

In some cases, not quite 1500 was supported, and reducing the frame size to slightly smaller, as you did, improved speed modestly.

I have never seen a complaint posted here that the system would not properly connect and run without reducing from the standard 1500 frame size.

here is a link to that older discussion about tuning frame size:

Mac Pro NIC not close to 1 Gbps connectio… - Apple Community

So the specs I can easily find suggest the DMSE is a fast router with no Wi-Fi capability.

and a U6 Pro is a ceiling-mounted Access point with Ethernet trunk that has four antennas for the 5 GHz band. it appears to require Power-over-Ethernet, which in some cases may limits its trunk speed.

Is that correct?

<< Have recently discovered that enabling a VPN on the Macs resolves the problem. >>

Enabling a VPN is not a solution to ANY problem known to mankind, except the requirement to be 'Present' on an institutional network to do your work. Almost all other uses are a complete SCAM.