Which ethernet cable do I need for Imac (2019) please

Mac ethernet ports have automatic switching so should work with regular or crossover cables.

About the external features and ports on iMac and iMac Pro - Apple Support

"The port automatically detects which type of device you connect. You don't need to use an Ethernet crossover cable to connect to other Ethernet devices."

It doesn't need to be an Apple cable -- in fact I don't believe that they even make or sell any ethernet cables. Any generic ethernet cable should work fine. The following has information about speed capabilities of Cat 5, Cat 6 etc cables: https://www.digitaltrends.com/computing/different-types-of-ethernet-cables-explained/

It may be that your internet service provider's modem needs to "accept" the iMac; when I had Comcast years ago, I had to call them if I had a new computer and they would enter the info on their end and then reset the modem. Once done, it would immediately recognize the iMac.

Ethernet plugs are universal and, according to Apple's specs, both 21.5 and 27" have ethernet slots.

As others have pointed out, a normal ethernet cable should / will work fine. However, there is an ethernet cable variant that you may have accidentally stumbled on in try to set up your ethernet connection. It is not a normal pass-through cable and will reverse the connection order of the individual connectors in the cable (in my company it used to be called an "ethernet modem cable"). Long story short, if one cable doesn't work, try another one. Even if the above is not what you are encountering, one cable may not work due to a damaged connector contact and another cable will.

Just a quick additional comment regarding the modem. If replacing the ethernet cable doesn't solve the problem, you might also reset your modem. Just turn it off and then back on, then see what happens.

+1 to pokey b ... even ethernet people forget about that second cable type.

Thank you... Regards, Pokey

Ah ha ! I was unaware of that, thank you. I had been (assumedly) tripped up by it years ago (20+), but then that might have been a defective cable(?).