Ethernet cable kills my internet

So Saturday morning I powered up my iMac (iMac (Retina 4K, 21.5-inch, 2017)) running OS Big Sur 11.1 and I find NO internet connection?

It was running fine and good on Friday night.

I depowered and depowered the iMac, still NO internet.

I checked other devices in the house and ALL have an internet connection.

I then start fault finding.

Switch from Ethernet connection to WiFi.

Cycle power still no internet

Power down

I then unplugged the Cat5/Ethernet cable and repower.

I now have the internet on the WiFi

Recycle power & still the internet on WiFi.

Reconnect the Cat5/Ethernet cable NO WiFi.

Unplug the Cat5/Ethernet and the internet back on WiFi.

I used the same cable and plugged the Cat5/Ethernet into my MacBook Air and internet access fine (Both Ethernet and WiFi).

I have tried many Cat5/Ethernet cables in both devices BUT always ending with NO internet on iMac with Cat5/Ethernet connect.

Any/All help would be appreciated.

Alan