Upfront Builder PHP error causing white screen of death

I recently upgraded my server from PHP 5.5 to 7.0, updated some packages and rebooted the server. After the upgrade, the website is now stuck in a white screen of death.

I briefly turned on display debug to see the error. It is: Fatal error: Uncaught Error: Class ‘DOMDocument’ not found.

I did a bit of research prior to this and understand that Upfront is no longer being developed, I would just like to have this issue resolved and have the site functional again. Please help.