Two wired connections created at startup but only one allows to reach the internet and needs manual activation

I have installed NixOS on a x86_64 machine on a nvme ssd where the mother board comes with an integrated network card (1x RJ-45 LAN Port). The nvme disk is btfrs encrypted and I have followed the guide on the wiki to remote unlock it at startup. Remote unlocking works (only tried inside LAN at the moment). However, after booting to the system, I can only ping devices in my LAN but not reach the internet:

myuser@server ~> ping google.com
ping: google.com: Name or service not known
myuser@server ~ [2]>

When giving nmtui via terminal and then going on Activate a connection, I get this screen:
2

Where this enp1s0 is the activated wired connection. I found out that in order to solve my issue (reaching both devices in my LAN and the internet), I have to activate what is called Wired connection 1. Right after this is activated, enp1s0 disappears from the list.

4

Do you know why I get this issue and how to solve it? So I would like to get only one wired connection and to reach out both my LAN devices and the internet.

I have attached some portions about the network that might be relevant from my configuration.nix:

  networking.hostName = "server"; 
  networking.networkmanager.enable = true;  

  services.tailscale.enable = true;
  services.tailscale.useRoutingFeatures = "server"; 
  
  users.users.myuser = {
    isNormalUser = true;
    extraGroups = [ "wheel" "docker" "networkmanager"]; 
  };

  networking.firewall.allowedTCPPorts = [ 22 ];
  networking.firewall.enable = true;
  networking.firewall.allowPing = true;

…and hardware-configuration.nix:

  boot.initrd = {
    availableKernelModules = [ "r8169" "xhci_pci" "ahci" "nvme" "usbhid" "usb_storage" "sd_mod" ];
    kernelModules = [ "dm-snapshot" ];
    network = {
      enable = true;
      ssh = {
        enable = true;
        port = 22;
        authorizedKeys = [ "ssh-rsa XXX" ];
        hostKeys = [ "/etc/xxx/initrd/ssh_host_rsa_key" ];
      };
      postCommands = ''
      # Automatically ask for the password on SSH login
      echo 'cryptsetup-askpass || echo "Unlock was successful; exiting SSH session" && exit 1' >> /root/.profile
    '';
    };
  };

  boot.kernelModules = [ "kvm-intel" ];
  boot.kernelParams = [ "ip=dhcp" ];

  networking.useDHCP = lib.mkDefault true;
  # networking.interfaces.enp1s0.useDHCP = lib.mkDefault true;