db.external.template

$TTL    60
$ORIGIN local.example.com.
@       IN      SOA     examplecom.ddns.net. admin.example.com. {{SERIAL}} 300 900 604800 60
@       IN      NS      examplecom.ddns.net.
@       IN      A       {{EXT_IP}}
*       IN      CNAME   examplecom.ddns.net.

gen-zone.js

import { readFileSync, writeFile, writeFileSync } from 'fs';
import fetch from 'node-fetch';
import { fileURLToPath } from 'url';
import { resolve, dirname } from 'path';

const __dirname = dirname(fileURLToPath(import.meta.url));

const { UDM_USERNAME, UDM_PASSWORD, UDM_HOST, NS1_IP, NS2_FQDN, EXT_IP } = process.env;
const DOMAIN = '';

const authenticate = async (host, username, password) => {
  const result = await fetch(`https://${host}/api/auth/login`, {
    method: 'POST',
    body: JSON.stringify({ username, password }),
    headers: {
      'Content-Type': 'application/json'
    }
  });

  const cookieHeader = result.headers.get('set-cookie');
  const { groups: { token } } = cookieHeader.match(/^TOKEN=(?<token>.*?);/);

  if (!token)
    throw new Error('h');

  return token;
};

const formatLines = lines => {
  if (!Array.isArray(lines) || lines.length === 0)
    return '';

  const cols = lines[0].length;

  const columnLengths = new Array(cols).fill(0);
  for (const line of lines) {
    if (line.length !== cols) throw new Error('Inconsistent data');
    
    for (let i = 0; i < line.length; ++i)
      columnLengths[i] = Math.max(line[i].length + 4, columnLengths[i]);
  }

  return lines.map(it => it.map((col, colIdx) => col.padEnd(columnLengths[colIdx])).join('')).join('\n');
};

const cleanName = name => name.replace(/[., _+|=]/g, '-').replace(/é/g, 'e').replace(/[^a-zA-Z0-9-]/g, '').toLowerCase();

const main = async () => {
  const token = await authenticate(UDM_HOST, UDM_USERNAME, UDM_PASSWORD);
  
  const deviceResponse = await fetch(`https://${UDM_HOST}/proxy/network/api/s/default/stat/sta`, {
    headers: {
      'Cookie': `TOKEN=${token}`
    }
  });

  const { data: rawDevices } = await deviceResponse.json();
  const filteredDevices = rawDevices.filter(it => (it.hostname || it.name) && it.ip);
  const devices = filteredDevices.map(it => ({
    cname: cleanName(it.name ?? it.hostname),
    mac: it.mac,
    ip: it.ip
  }));

  const configurationResponse = await fetch(`https://${UDM_HOST}/proxy/network/api/s/default/rest/user`, {
    headers: {
      'Cookie': `TOKEN=${token}`
    }
  });

  const { data: rawConfigurations } = await configurationResponse.json();
  const filteredConfigurations = rawConfigurations.filter(it => it.use_fixedip && it.fixed_ip && !devices.find(it2 => it2.mac === it.mac));
  devices.push(...filteredConfigurations.map(it => ({
    cname: cleanName(it.name ?? it.hostname),
    mac: it.mac,
    ip: it.fixed_ip
  })));

  const nameMap = {};
  for (const { cname, mac } of devices) {
    if (!(cname in nameMap))
      nameMap[cname] = [];

    nameMap[cname].push(mac);
  }

  for (const value of Object.values(nameMap))
    value.sort();

  for (const device of devices) {
    const { cname, mac } = device;
    console.log
    if (!(cname in nameMap) || nameMap[cname].length === 1)
      continue;

    const order = nameMap[cname].indexOf(mac);
    device.cname = `${cname}-${order}`;
  }

  const deviceLines = devices.map(it => [it.cname, 'IN', 'A', it.ip]);
  const tsStr = `${Math.floor(Date.now() / 1000)}`;
  const lines = [
    [`${DOMAIN}.`, 'IN', 'SOA', `ns1.${DOMAIN}. admin.example.comF. ${tsStr} 300 900 604800 60`],
    [`${DOMAIN}.`, 'IN', 'NS',  `ns1.${DOMAIN}.`],
    [`${DOMAIN}.`, 'IN', 'NS',  `${NS2_FQDN}.`],
    ['ns1', 'IN', 'A', NS1_IP],
    ...deviceLines
  ];

  const forwardZone = `$ORIGIN ${DOMAIN}.\n$TTL 60\n${formatLines(lines)}\n`;

  const octet = (ip, num) => ip.split('.')[num];

  const makePtrLine = device => [octet(device.ip, 3), 'IN', 'PTR', `${device.cname}.${DOMAIN}.`];

  const rev1DeviceLines = devices.map(makePtrLine);

  const rev1Lines = [
    ['@', 'IN', 'SOA', `ns1.${DOMAIN}. admin.example.com. ${tsStr} 300 900 604800 60`],
    ['', 'IN', 'NS', `ns1.${DOMAIN}.`],
    ...rev1DeviceLines
  ];
  const reverseZone1 = `$TTL 60\n${formatLines(rev1Lines)}\n`;

  writeFileSync('db.192.168.1', reverseZone1);
  writeFileSync('internal.db.local.example.com', forwardZone);

  const externalZone = readFileSync(resolve(__dirname, 'db.external.template'), { encoding: 'utf-8'}).replace('{{EXT_IP}}', EXT_IP).replace('{{SERIAL}}', tsStr);
  writeFileSync('external.db.local.example.com', externalZone);
};

main();

Variables:
UDM_USERNAME The username for the UDM Pro.
UDM_PASSWORD The password for the UDM Pro.
UDM_HOST The hostname for the UDM Pro.
NS1_IP The internal IP of the primary nameserver.
NS2_FQDN The external FQDN (e.g., ns2.example.com) of the secondary nameserver.
EXT_IP The public IP of the private network.