iMac loses connection to the ethernet if I don't use it for a while

First restart in Safe Mode. If that does not work, please answer do you have any third party:

• Antivirus apps installed?
• Cleaning apps installed?
• VPN apps installed?
• Security installed?
• Maintenance apps installed?

Renewing the least should not be interrupting your connection. Your router reexamines those computer attached and reassigns new IP numbers as needed if set up to do that. In a busy office environment, with users coming and going, a renewed lease clears out the no longer active users. But if you're still connected, you should stay connected.

The greater the number of "temporary" users the shorter the lease should be. A home environment with few "temporary" users can get away with longer lease times because the router doesn't "fill up".

https://epb.com/get-connected/gig-internet/what-renew-lease-mean-on-wifi/

First, check your router. It may need to be rebooted. Disconnect it from power for about 5 minutes, then reconnect power. See if that fixes things.

If not, then check all your connections. Unplug the ethernet cable from your Mac & from the router ... and any other connectors in between. Then plug them back in. (For example, in my case, my Mac connects to an ethernet port in the wall, which in turn runs to a distribution box in the basement, which in turn is connected to an ethernet switch, which is connected to my router - 5 connections in all. 6 if you also count the router's connection to my FiOS ONT. Any of the connections could get loose or dirty over time.)

If neither approach fixes the problem, let us know.

AHCMac wrote:

I have 2 routers with the second downstream from the first.

I'm glad you were able to straighten out the settings. Having 2 routers, one downstream from the other, is often a source of internet connection issues. For future reference, it would be good to keep a record of the changes you made in your routers' & iMac's settings.

MartinR wrote:

Having 2 routers, one downstream from the other, is often a source of internet connection issues.

👍

You definitely do not want two routers providing routing services on your network. That will cause a conflict resulting in the problems you described.

Unless you have a specific networking requirement to have two routers (performing as NAT routers) in series, then the downstream router should be reconfigured as a bridge. This would disable the NAT function in that downstream router to avoid having a "Double NAT" condition ... which would primarily affect any network client connected to that downstream router for Internet connectivity/data throughput performance.

AHCMac wrote:

Thanks for the comment. The second router is a slave to the first, so only one is setting the service.

Be sure to configure it as a bridge. I think that's what you intend but using the correct terminology will prevent confusion.

You sound like an ideal candidate for a Mesh network to get coverage all over your home rather than using 2 routers or a bridge. If you are not familiar with Mesh networking look into them and you will see what I mean when I say you are an ideal candidate. I had a similar problem with coverage in our home which is a 1 story home but it is long and the original router was on one end of the home, that meant very poor coverage on the opposite end of the house. I tried setting up a bridge which was a total waste of time and money. However when I installed a Mesh network, the problem went away and now I even have great coverage all over the house, the yard (front and back) so I can set up wireless devices like smart plugs to manage things like outdoor fountains, lights and now I have great coverage for Sonos outside wireless speakers. Do yourself a big favor and look into a Mesh network and install it. You won't regret it.