U5226KW — Built-in 2.5GbE and USB Hub Not Working Properly on macOS (MacBook Pro Apple Silicon, Firmware M2T103)

Hey all. Long post but bear with me.

I just spent an entire evening troubleshooting what I thought was a cable problem on my brand new U5226KW, however, now I believe it to be a firmware issue. Here's everything I found.

SETUP:
- Dell U5226KW, Firmware M2T103 (latest at time of writing)
- MacBook Pro (M4 Max on Tahoe), single (included with monitor) Thunderbolt 4 cable
- 3Gbps symmetric Google Fiber, UniFi network with 2.5GbE switch ports

THE PROBLEM:

The monitor's built-in 2.5GbE ethernet is basically broken on macOS. I'm getting ~500Mbps down but ~2300Mbps up on a 3Gbps symmetric connection. Upload is fine, download is terrible. Ran this on multiple speed test services to confirm.

Second issue: ALL downstream USB ports on the monitor — every USB-A and USB-C port on the back AND the pop-out ports on the bottom — are capped at 5Gb/s. They're advertised as 10Gb/s. Half speed across the board.

WHAT I RULED OUT:

I spent hours on this. It's not the cables, not the switch, not the Mac, not the network. Every other device on my network gets full symmetric 2.5GbE. The Mac itself is fine — I tested it with multiple ethernet adapters plugged directly into the Thunderbolt port and got full symmetric speeds every time.

WHAT'S ACTUALLY HAPPENING:

Dig into macOS System Information and it tells the whole story. When the monitor's built-in ethernet is active, macOS is loading the driver com.apple.driver.usb.cdc.ncm — a completely generic CDC-NCM driver that has nothing to do with the RTL8156 chip Dell used for the ethernet. When I plug a USB ethernet adapter directly into the Mac's Thunderbolt port, macOS loads the correct Realtek driver and I get perfect symmetric speeds.

The monitor's USB hub is presenting the internal ethernet chip incorrectly to macOS over Thunderbolt. That's why the wrong driver loads, and that's why download is wrecked while upload is fine. Same mis-enumeration issue is also what's capping the USB ports at 5Gb/s instead of 10.

This isn't a Mac issue or an Apple driver issue. It's the monitor firmware mis-handling USB device enumeration over Thunderbolt.

PRECEDENT:

This is identical to the bug Dell fixed for the U2725QE and U3225QE (KB ID: 000356362) where the USB hub was falling back to slow speeds on Thunderbolt-connected Macs. Dell engineering reproduced it and fixed it with firmware M2T102 in August 2025. That fix came about 4 months after users started formally reporting it.

The U5226KW has the same issue. I'm on the latest firmware M2T103 and it's not fixed.

ASKING FOR:

Dell engineering to look at this the same way they looked at the U2725QE issue. The built-in 2.5GbE and 10Gb/s USB hub are two of the main reasons to spend $2,899 on this monitor. Neither is working correctly on macOS right now.

Happy to share System Information screenshots, speed test results, or ioreg output if it helps the engineering team.

Is anyone else seeing this? Would love to know if other U5226KW Mac users are in the same boat.