For anyone else wondering, this is how I got it to work. The following is my /etc/apache2/sites-available/000-default.conf doing a reverse proxy.
<VirtualHost *:443>
ServerName domain.tld
ProxyPass /
http://xxx.xxx.xxx.xxx:xx/
ProxyPassReverse /
http://xxx.xxx.xxx.xxx:xx/
RedirectMatch ^/$
https://domain.tld/ SSLEngine on
SSLCertificateFile /etc/ssl/path-to/domain.crt
SSLCertificateKeyFile /etc/ssl/path-to/domain.key
LogLevel info ssl:warn
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>