Yes. Within the Forms database, go into the "cf_options" table and edit the "FormsHost" value to be the external Forms Server URL. The email reminder uses the URL from that table value, then adds the specific instance information to the end of it.

The format should be:  "https://<your_external_URL>/Forms/"

There is another value in that table called "FormsHostEmailOverride" which is kind of confusing, as I thought this would override the URL in reminder emails, but testing has proven that is not the case. However, as a best practice, I usually set the URL value to be the same for both of those options (FormsHost & FormsHostEmailOverride).