Mac os High Sierra 10.13.6

Replacing an older Linksys router with a Nighthawk AX3000.  Linksys worked fine just wanted more range and faster 2.4 ghz for my Roku streamers.

I have Spectrum cable internet 200mbps.

I have Alexa dots, Ipads, Iphones, Rokus and an Imac that all connected wirelessly just fine.

I have an older Imac late 2009 running High Sierra 10.13.6 with all updates that can only connect through ethernet and I have all 5e cables.  Nighthawk would not recognize this computer.  

I have a printer, an audio receiver and NAS all connected through ethernet just fine so my cables are working.  I switched around the cables to different ports on the router just in case the router had a bad port.  All ports on the new router seen to be working fine.  Installed with the app then configured manually shut down started back up in the right sequence, did a reset....tried everything I know to do.

Was on the phone with tech support and she had me setup DHCP manually and enter IP, susbnet mask etc.  Still would not recognize the hardwired IMac.  We finally gave up.  I connected the Linksys back up and everything connected just fine.  How can I get this router to recognize the hardwired IMac?  Everything else connected as expected its just this one desktop that won't.  I am no expert but I have setup a dozen routers for myself, friends and family over the years.  Everything was typically plug and play.  Please help. I don't use advanced fetaures on the router as I don't need to.