We are trying to set up dual routing per these instructions for our domain. We need mail to flow to our internal server as well as to our GSuite for Education domain. Here are the instructions we were given.

Route mail to another server

You've chosen to activate Gmail by routing dcbe.org's email through a mail server that's not hosted by Google. This might be your own on-premise server, or a third-party email service (say, that provides email archiving or spam protection).
To do this:
Don't change your domain's mailflow to point to our servers. Continue routing mail to your current mail server (or whichever server hosts the add-on service you want to use).

For your reference, the current MX records for dcbe.org are:

Priority Points to

Configure that mail server to route mail to ASPMX.L.GOOGLE.COM.
Once this change goes into effect, mail will flow through your server to ours, allowing you and your users to send and receive messages via your Gmail accounts.
When you're ready, Next to move on to the next step.

We aren't sure where to put the ASPMX.L.GOOGLE.COM entry in our mail server settings.

Can anyone please point us in the right direction?

Thanks so much,