Change php version to php7.3 for Buster.
apt install libapache2-mod-fcgid php7.0-fpm apt purge libapache2-mod-php7.0
a2dismod mpm_prefork a2enmod mpm_event actions fcgid proxy_fcgi alias setenvif ssl rewrite a2enconf php7.0-fpm systemctl restart apache2
Create file phpinfo.php in DocumentRoot with this content:
<?php phpinfo( ); ?>
then visit http://yoursite.com/phpinfo.php
Then line with “Server API” should have “FPM/FastCGI”.