$ lsb_release -a
No LSB modules are available.
Distributor ID: Raspbian
Description:    Raspbian GNU/Linux 10 (buster)
Release:        10
Codename:       buster

#include "datalink.h"
#include <WiFi.h>

WiFiServer server(23);
WiFiClient client;

void setup() {
  Serial1.begin(115200);
  WiFi.config(LOCAL_IP, GATEWAY, SUBNETMASK, PRIMARY_DNS);
  WiFi.begin(SECRET_SSID, SECRET_PASS);
  while (WiFi.status() != WL_CONNECTED) {
    Serial1.print(".");
    delay(100);
  }
  server.begin();
}

void loop() {
  client = server.available();
  if (client) {
    while (client.connected()) {
      if (client.available()) {
        Serial1.write(client.read());
      }
    }
    client.stop();
  }
}

#include "datalink.h"
#include <WiFi.h>

WiFiServer server(23);
WiFiClient client;

void setup() {
  Serial1.begin(115200);
  WiFi.config(LOCAL_IP, GATEWAY, SUBNETMASK, PRIMARY_DNS);
  WiFi.begin(SECRET_SSID, SECRET_PASS);
  while (WiFi.status() != WL_CONNECTED) {
    Serial1.print(".");
    delay(100);
  }
  server.begin();
}

int getClientChar() {
  while(1) if(client.available()) return client.read();
}

void loop() {

  static int c;
  client = server.available();
  if (client) {
    while (client.connected()) {
      if (client.available()) {
        c = getClientChar();
        if (c==0xFF) {
          client.write(c);
          c = getClientChar();
          client.write(c);
          switch(c) {
            case 0xF0:
              break;
            case 0xFA:
              for(int i=0;i<5;++i) client.write(getClientChar());
              break;
            default:
              client.write(getClientChar());
              break;
          }
        } else {
          Serial1.write(c);
        }
      }
      if (Serial1.available() > 0) {
        client.write(Serial1.read());
      }
    }
    client.stop();
  }
}