If your HTTP/2 setup with Apache is not working properly you should check your Apache log files first. If you encounter an error message like:
[Wed Mar 27 18:05:15.415380 2019] [http2:warn] [pid 25839] AH10034: The mpm module (prefork.c) is not supported by mod_http2. The mpm determines how things are processed in your server. HTTP/2 has more demands in this regard and the currently selected mpm will just not do. This is an advisory warning. Your server will continue to work, but the HTTP/2 protocol will be inactive.
[Wed Mar 27 18:05:15.425720 2019]
you can fix the issue by installing the php-fpm package. The following tutorial works for Ubuntu 18.04 LTS but should also work for older Ubuntu versions like 16.04 LTS
apt-get install php7.1-fpm # You might need to change the PHP version to your PHP version
a2enmod proxy_fcgi setenvif
a2enconf php7.1-fpm # Check the PHP version
a2dismod php7.1 # This disables mod_php. Check the PHP version here too
a2dismod mpm_prefork # This disables the prefork MPM. In the following you will switch to php-fpm
a2enmod mpm_event # Enable event MPM.
Your server should be ready for HTTP/2 now.