I want to use next env in next.config.js assetPrefix: ASSETS_CDN_URL

I am loading this code to load env in next.config.js

const {loadEnvConfig} = require('@next/env');
loadEnvConfig(process.cwd());

const isProd = process.env.NODE_ENV === 'production';
const ASSETS_CDN_URL = process.env.NEXT_PUBLIC_ASSETS_CDN_URL;
console.log('from next.config.js', ASSETS_CDN_URL);

I got output in my terminal

info  - Loaded env from /home/ubuntu/frontend/.env
from next.config.js https://cdn-server2.cdn.com

But it fails to load with cdn. If I give value in direct string like this assetPrefix: 'https://cdn-server2.cdn.com' It works perfectly, I have two servers I have two cdn server for this, How can I resolve this issue? Now I have to manually set the server in static , that is full manual. How can i load assetPrefix from my env?