Connecting Airport Express to internet via Ethernet

I have an Airport Express in an outside room - connected via ethernet cable to an Airport Extreme in the main house. (I have another Express doing the same job in a different part of the house off the same Extreme)

The purpose of the Express is to provide good seamless wi-fi and internet connectivity in the outside room. It is set up to 'create a wireless network' using the same network name, password and security as the Extreme inside. It worked perfectly for months until suddenly it stopped. (Amber light. No internet connection)

Cycled power and no change.

Next assumption was faulty Express. So I took the Express inside and connected it to another ethernet cable direct from the Extreme and it worked fine.

So then I assumed it must be the cable to the outside room. I therefore ignored the Express and plugged the ethernet cable directly in to my Macbook via thunderbolt, turned off wi-fi on the macbook and was able to get an internet connection via thunderbolt and the ethernet cable without problems. Cable seems OK then?

Baffled now.

Tried resetting the original Express. No luck.

Tried a second Airport Express in the outside room. Didn't work. Same issue

I cant understand why the Express works using one cable but then not with a different one in a different place

Is it possible that a faulty ethernet cable could work for my Macbook, but not for the Express?

Is the relative positioning of the extreme and the airport expresses relevant?

Any help would be appreciated. Replacing the ethernet cable is a last resort and will avoid lifting paving etc.

Thank you