Tested with DSMR v2.2 meter, Raspbian GNU/Linux 11 (bullseye)

• Connect serial to DSMR with a level shifter
• Test serial connection

  stty -F /dev/ttyAMA0 9600 cs7 -cstopb parenb
  cat /dev/serial0

• Install ser2net

  apt install ser2net

• Configure in /etc/ser2net.yaml

  connection: &con0
      accepter: tcp,23
      enable: on
      options:
        max-connections: 10
        telnet-brk-on-sync: true
      connector: serialdev,
                /dev/ttyAMA0,
                9600e71,local

• This configuration did not open port 23 at reboot. It seems the service starts before the network is available and that didn't work. Edit the ser2net.service file

  systemctl edit ser2net.service
  
  # Add the following lines:
  [Unit]
  After=network-online.target
  Wants=network-online.target

• Restart ser2net