Oddly specific 2.5gbps ethernet / ASUS ET12 Pro issue

[MikkyX]

My setup is as follows:

• M1 Mac Mini running latest version of Ventura
• Anker PowerExpand USB-C to 2.5gbps ethernet adaptor
• ASUS ET12 Pro Router (provided by ISP)
• Cat6 ethernet cable connecting adaptor to router via it's 2.5gb LAN port

I have 2gbps each way internet and I wanted to upgrade my Mac Mini (which only has a 1gbps port) to take full advantage - so I bought the above adaptor. Alas, this combination does not seem to want to work. When the Mac Mini is connected through the adaptor via a Cat6 cable to the router's 2.5gb LAN port, the lights on the adaptor blink on and off every few seconds and you can see "USB 10/100/1G/2.5G LAN" bouncing up and down the list in Network but never becoming connected.

Take any part of it out of the equation and things seem fine. Here's what I've tried so far - keeping the rest of the setup (so adaptor, with cat6 cable, into 2.5gb LAN port unless otherwise stated):

1. Replace the Mac Mini with iPad Pro 5th Gen (iOS 17) = full speed ahead
2. Replace the Mac Mini with a 2019 Intel MBP (also on Ventura) = full speed ahead
3. Replace the Mac Mini with a Windows laptop = works, but not at 2gbps+
4. Replace the Mac Mini with a Pixel 7 Pro = works, but not at 2gbps+
5. Connect the cable from the Mac Mini's own ethernet port to the router's 2.5 LAN port = works at 1gbps
6. Connect the cable from the adaptor to the router's standard LAN ports = works at 1gbps

So any other combination of hardware seems to work, but not the specific combination I'm looking for. It doesn't feel like an OS issue because it works on another Ventura machine. It's not an Apple issue because it works on iOS. It's not a router or port issue because it works on other devices. It's not the Mac because if I switch the port I connect to on the router, it works.

Ultimately it only seems to fail when it's the specific setup I'm actually trying to achieve - and I'm clueless as to why. It feels like it could be a conflict between the Mac and router when they're trying to negotiate a connection of that speed? Not sure how to fix it and would appreciate any pointers on what else to try / where to start looking. Thanks