Latency Test OK, dennoch Fehlermeldung

tommylight, thanks for your answer! I'm sorry that I dont have read all of the relevant manuals and HowTo's so that I did not follow instructions that I did not need until some days ago. As a newbie in Linux and LCNC, I needed weeks and months to get to a working machine. Until then, everything worked fine, both networks in parallel, the only problem was the latency that I did not understand. PCW brought me up to here, we had a solution in sight, and I was not mentioning that I'm doing sth wrong.
So, I will follow your advice tonight and give feedback soon.

@tommylight - can you please tell me where to find that network HowTo? Couldn't find that here.
After deleting and reboot the network manager shows no wired connection anymore, no auto inet. I can send the actual ip a if my internet is working again. Actually it isn't. I'm writing from my windows PC.

@Chris@cnc - i changed the network/interfaces to your suggestion and after reboot I dont have any connection no more.

Is that allright? Do I have to reconfigure both connections as before? How do I find out which name is what card? I want to have the internal intel card for MEsa and the RTL8111 for Internet. Cables are sitting ok for that config.
So, I tried to configure a new connection for Internet with eno1 - my settings in network manager are set, but no connection is shown. ip a shows the new entry after reboot, but there's no connection shown in the upper right edge of my desktop.

Thanks in advance!

What version of LinuxCNC and OS are you using?

 I can try an duplicate this. I don't have a PCI Ethernet card
compatible test system but can test with  a USB-->Ethernet
dongle for general internet access, This should be similar
enough to at least get an idea where things are going wrong.
 

Hi PCW!
I am working with Debian 12 and Linuxcnc 2.9.1

I don't think I can get LinuxCNC 2.91 but 2.93+Debian12
should be identical as far as network setup goes.

So, my last actions today summarized:
I deleted all wired connections.
I triet to find out which connection goes to what hardware. Cable put out from the MESA card, eno1 is definitely the internal Intel card, ip a shows "no carrier" on eno1, so that is quite clear and how I want it. eno1 has had much better pings when I did the sudo ethtool -C eno1 rx-usecs 0.
Confusion about that resolved.

enp2s0 is the RLT8111 card. I want to get Internet over this connection.
this means, I have to do what?

Q: Further configuration only in the etc/network/interfaces? Not in the network manager? Seems to be quite more complex to do all settings just in the /interfaces file. (newbie in Linux)  I can't find the doc where either/or is defined.

Confusion exists further about this:
Mesa Card has 10.10.10.10, Intel 82579LM card has 10.10.10.2    What is 10.10.10.1?

Suggestion in this thread was:
"auto eno1
       iface eno1 inet static
           address 10.10.10.2
           hardware-irq-coalesce-rx-usecs 0"

Suggestion in the google doc from PCW is:
"sudo geany /etc/network/interfaces

to append the following at the end of the file:

     auto enp2s0iface
          enp2s0 inet staticaddress 10.10.10.1 
         hardware-irq-coalesce-rx-usecs 0 
       Note, the last line is only required for Intel network cards."
Q: Besides the other connection enp2s0, what is the right configuration - 10.10.10.2 or .1 ?

Q: Do I need to have the loop in /interfaces to 127.0.0.1?

Thanks for any answers!