How to fix network connectivity issues on iMac

From the MacBook, enter the following command in the Terminal app: nc -z <IP address of iMacE> 1-1024

The result should like something like the following:

Now enter the same command with iMacW.

For File Sharing, the last two items will be critical. Please post back your results.

You also mentioned that iMacW was connecting to a repeater. You really need to provide us with the exact configuration of your local network, including all of the networking hardware that is being used (that is in addition to the TP-Link.)

Ok, the "missing" port that needs to be open is port 139. That port lets two computers establish a connection that is required for Finder to "see" that the iMacE has file sharing enabled.

This may be because, your network is not properly configured to allow for, as a minimum, the IPv6 "link-local" only protocol being enabled ... or it is being blocked.

Ok, since you have already renamed your two iMacs, let's concentrate on the network settings on both.

For clarity, I will name the Ethernet-connected iMac, as iMacE, and the wireless-connected one: iMacW.

Let's start with iMacE.

1. Go to System Settings > Network > Ethernet > Details ... > TCP/IP
2. Select Renew DHCP Lease.
3. Give it a few seconds, then check the following settings: Configure IPv4 = Using DHCP. Note the values for IP Address, and Router Address. These addresses should start with one of the following: 192,172, or 10. If, instead, it starts with 169, this Mac is not "seeing" the DHCP server in your TP-Link router.

Next, let's work with iMacW. The steps will be similar to what we did for iMacE:

1. Go to System Settings > Wi-Fi > Select your Wi-Fi network > Details ... > TCP/IP
2. Select Renew DHCP Lease.
3. Give it a few seconds, then check the following settings: Configure IPv4 = Using DHCP. Note the values for IP Address, and Router Address. These addresses should start with one of the following: 192,172, or 10. If, instead, it starts with 169, this Mac is not "seeing" the DHCP server in your TP-Link router.

For both iMacs, the IP Address of each should be in the same IP range. For example, if your TP-Link's DHCP server is serving IP address in the 192.168.0.x range, each Mac should be in the same range, but with a different last octet. The Router Address should be the same for both ... and pointing back to the LAN-side address of the TP-Link.

If all of this is good, I would like for you to use the Ping command in the Terminal to "ping" between these two iMacs to see if there are any issues with them communicating with each other. I would like for you to ping them from both directions. That is, from iMacE to iMacW, and then, from iMacW to iMacE.

To do so, starting with either iMac, run the Terminal app which is located in the /Applications/Utilities folder. At the Terminal's command prompt, enter: ping -c3 <enter the IP address of the other iMac>, and then press return.

A successful ping would have 3 transmitted & 3 pings received with 0% packet loss. Anything else would indicate an issue with your network. Please repeat this from the other iMac.

It should look something like the following:

Please post back your results.

If enabled, temporarily disable the Apple software firewall on all of your Macs. See if they can "see" each other now for file sharing.

What version of macOS is running on each Mac?

Incorrectly-configured subnets when multiple paths (Wi-Fi with intervening routers with mixes of wireless and wired) can cause connectivity issues, and buggy or failing routers (check for router firmware updates) can also cause issues. If there’s more than one Wi-Fi router present (and configured as a router, not as an access point), that can play havoc with connectivity when subnets are not configured. For many such configurations, switching routers over to APs can help. (Some routers can handle Wi-Fi and wired well, others not so much.)

How many IP network routers are present? Just that one TP-Link Archer AC1900?

Or is there another Wi-Fi router, or maybe an ISP IP router/firewall/gateway box involved?

Current firmware for that TP-Link Archer AC1900 is Archer AC1900(USW)_V1.6_1.13.2 Build 230824 from November 2023. (Details). If you’re not running that, update.

Wi-Fi interference can also be a potential issue. The built-in Wi-Fi troubleshooting tools are adequate and have the benefit of being trivial, while tools such as WiFi Explorer app can make troubleshooting shooting easier.

To view the basic Wi-Fi environment characteristics, Option-Click on the Wi-Fi logo 🛜 in the menu bar, and capture some Wi-Fi network data. Post the items shown in the following image including the Security mode and from Channel to NSS inclusive (Security, Channel, Country, RSSI, Noise, Tx Rate, PHY mode, MCS, NSS, green highlight) (without posting the Wi-Fi address, any local HotSpots that might be present, the network name, or the BSSID, all of which are expurgated from the image shown below, red highlight), and we can take a look at the basic network environment.

Was this always an issue or just started happening recently?

Regardless, what have you tried, so far yourself, to resolve this so that we don't offer potential solutions that may not have already worked for you?

It would also help if you can provide us with some details about the network that you have all of these devices connected to? Like the make & model of the wireless router or gateway, and how each of these devices are connected to it ... by Ethernet, or wireless, or both.

Ok, you only provided one of the answers to my questions. How about the other two? What you have done, and your network information.

Having the same computer name may be problematic, but want would be more so, would be if both iMacs have the same IP address.

Do you have any software firewalls running on any of these Macs? Anti-virus apps? Can all of these Macs access the Internet successfully?

I concur with hcsitas that you should restart your network router.

IPv6 "Link-local" only would be the minimum setting for all of your network devices, including your router. If they are already configured for IPv6 "Automatically" that will be fine.

... and, if that is the case, I'm not sure what is blocking communications on port 139 on your network. This is why I had asked if any of your Macs have an active software firewall running on them.

... and, if they are all already disabled, I'm not sure what the issue, or solution to offer you without much more extensive network troubleshooting.

What re the IPs of each?

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