GSC URL inspection: URL is not available to Google for Angular SSR app

Question

I have an Angular SSR app, which is hosted in DigitalOcaen. After submitting the sitemaps to GSC, this was the result for Page indexing:

The reasons mostly are these:

1. Server error (5xx)
2. Discovered - currently not indexed

When I tried to manually inspect URL, this was the result:

And when I tried to request indexing, this was the result:

This is my code for server.ts for the SSR app:

import 'zone.js/dist/zone-node';
import { ngExpressEngine } from '@nguniversal/express-engine';
import express from 'express';
import { join } from 'path';

import { AppServerModule } from './src/main.server';
import { APP_BASE_HREF } from '@angular/common';
import { existsSync } from 'fs';
import ssrForBots from 'ssr-for-bots';

export function app(): express.Express {
  const server = express();
  const distFolder = join(process.cwd(), 'dist/app/browser');
  const indexHtml = existsSync(join(distFolder, 'index.original.html'))
    ? 'index.original.html'
    : 'index';
  const defaultCrawlerOptions = {
    prerender: [
      'bot',
      'googlebot',
      'Chrome-Lighthouse',
      'DuckDuckBot',
      'ia_archiver',
      'bingbot',
      'yandex',
      'baiduspider',
      'Facebot',
      'facebookexternalhit',
      'facebookexternalhit/1.1',
      'twitterbot',
      'rogerbot',
      'linkedinbot',
      'embedly',
      'quora link preview',
      'showyoubot',
      'outbrain',
      'pinterest',
      'slackbot',
      'vkShare',
      'W3C_Validator',
    ],
    exclude: ['.xml', '.ico', '.txt', '.json'],
    useCache: true,
    cacheRefreshRate: 86400,
  };

  server.use(ssrForBots(defaultCrawlerOptions));

  server.engine(
    'html',
    ngExpressEngine({
      bootstrap: AppServerModule,
    })
  );

  server.set('view engine', 'html');
  server.set('views', distFolder);

  server.get(
    '*.*',
    express.static(distFolder, {
      maxAge: '1y',
    })
  );

  server.get('*', (req, res) => {
    res.render(indexHtml, {
      req,
      providers: [{ provide: APP_BASE_HREF, useValue: req.baseUrl }],
    });
  });

  return server;
}

function run(): void {
  const port = process.env.PORT || 4000;

  const server = app();
  server.listen(port, () => {
    console.log(`Node Express server listening on http://localhost:${port}`);
  });
}

// Webpack will replace 'require' with '__webpack_require__'
// '__non_webpack_require__' is a proxy to Node 'require'
// The below code is to ensure that the server is run only when not requiring the bundle.
declare const __non_webpack_require__: NodeRequire;
const mainModule = __non_webpack_require__.main;
const moduleFilename = (mainModule && mainModule.filename) || '';
if (moduleFilename === __filename || moduleFilename.includes('iisnode')) {
  run();
}

export * from './src/main.server';

Please share if you're aware of any solutions, thank you.