Problem connecting Pi4 via GENET

Those with RCBOs … Powerline adapters will work fine

Indeed, I have RCBOs throughout and powerline is fine. I haven’t done a speed test, but it’s never seemed slow.

It’s not just older homes impacted by poor wifi – modern houses with reinforced concrete floors get no signal whatsoever through them, for wifi or zigbee. Thankfully the developers here put in cat5 to each floor (though zigbee is still awkward with repeaters at the bottom and top of each flight of stairs).

Those with RCBOs, if they’re comliant with IEC 61000-4 which all should be, Powerline adapters will work fine with them, if they dont, they’re most either not compliant or if they were tested would beclassed as faulty.

Anything that trips on residual current, whether or not it also trips on overcurrent, does not have a direct connection to the neutral busbar – the residual current transformer is in the way. The leakage inductance isolates the neutral of each circuit from all other circuits, at the frequencies that the modems are using for communication. That’s the problem in my house, and it’s quite a common one.

If I could access two sockets on the same breaker, one with easy patch cable access to the router and the other with easy patch cable access to the Ethernet switch upstairs, there wouldn’t be a problem. But I can’t.

After setting it back auto, egeinfo now shows it operating at the expected speed of 1000baseT full duplex

Auto is pretty much the best setting to be using, not least because it then matches the capabilities of the other end in speed¹ terms.
The duplex can give issues if the other end is nailed to full-duplex, as that fixed full-duplex suppresses the auto-neg communication that the auto on the other end expects and that then defaults to half-duplex. A duplex mismatch will give awful performance, single digit percentage of the nominal speed being common.

¹ The negotiation has two components – speed 10/100/1000² and duplex.
1000 will require all eight cores with good connectivity (otherwise it drops to 100) as well as similar speed capabilities in the hardware at both ends.