Add this to .htaccess
ref. – https://wordpress.org/support/topic/serve-static-assets-with-an-efficient-cache-policy/
Plugin – not required though – https://wordpress.org/plugins/http-headers/
# Cache Control - google page speed - Serve static assets with an efficient cache policy
<FilesMatch "\.(js|css|jpg|gif|png|pdf|swf|svg|svgz|ico|ttf|ttc|otf|eot|woff|woff2|webp)$">
<IfModule mod_headers.c>
ExpiresActive On
ExpiresDefault "access plus 1 month"
Header set Cache-Control "public, immutable, max-age=2628000, s-maxage=2628000"
</IfModule>
</FilesMatch>