Nuxt.js: can't generate routes

I want to generate both static routes (/contact, /about, …) and dynamic routes (/project/1, /project/2, …) for my project, so that when user refreshes the page while visiting any of these routes, the page still works.

But when doing npm run generate I only get Generated route "/" and in /dist folder I see no routes generated.

Nuxt.js version used: 2.14.7

I tried with both universal and spa modes, it works with neither.

In nuxt.config.js I have at the top:

const axios = require('axios')

const dynamicRoutes = async () => {
  const routes = await axios.get('http://my-project.com/wp/wp-json/projects/v1/posts')
    .then(res => res.data.map((project) => `/project/${project.ID}/${project.post_name}`))
    .then(res => res.concat(
      [
        '/about',
        '/contact',
        '/portfolio'
      ]
    ))
  return routes
}

Then in export default {}:

generate: {
  routes: dynamicRoutes
},

generate: {
 async routes(){
   const routes = await axios.get('http://my-project.com/wp/wp-json/projects/v1/posts')
    .then(res => res.data.map((project) => `/project/${project.ID}/${project.post_name}`))
    return [...routes,
      [
        '/about',
        '/contact',
        '/portfolio'
      ]
    ]
  }
}