Ok, installed the first master with this command:

curl -sfL <https://get.k3s.io> | INSTALL_K3S_VERSION=v1.25.2+k3s1 K3S_TOKEN="REDACTED" sh -s - server --cluster-init --cluster-cidr "10.44.0.0/16" --flannel-iface "enp35s0.4000" --node-ip "10.45.0.1" --node-external-ip "95.217.198.219"

It’s up and running fine.