On iOS 13.7+ you can now side-load your EN Server configuration before submitting to Apple.
(Source document)

This will allow you to retrieve and submit exposure keys to a Key Server on your local device.

Generating a Config File

Starting with the sample test configuration.

Modify it by entering the desired bundle ID, public key, etc. (see keys “appBundleId”, “publicKeyVersion”, “publicKey”).

Loading a config through developer menu

Enable the developer menu

If the Settings → Developer menu isn’t available, connect the phone to a computer running Xcode, and view the phone via Window → Devices in order to add the Settings→Developer

Install profile that enables Developer -> Exposure Notification

Transfer the ExposureNotification-DeveloperSettings to your phone.  If you click on a profile in an email, message, or the Files app, you will get a message asking you to chose a location to install it, and a message that you have to go to settings to install it. 

Then, go to Settings to install the profile you downloaded above. 

Get an appropriate config file in plist format

Normally, config files are stored in json format. The developer menu path requires that config files be in plist format. Fortunately, it is easy to convert a json config file to a plist config file.

To convert from .json format to .plist format use:

plutil -convert xml1 in.json -o out.plist

Put the file somewhere it can be accessed on the phone and seen by the Files app. If you have the file on a macOS computer, you can share it to the iPhone, and save it in the Files apps. 

Load the Config

Go to Settings → Developer → Exposure Notifications

Enable Developer Server Configuration

Choose override Configuration File

• This should generate a notification that says “Successfully set override configuration file”

Go to Settings → Exposure Notification

Turn on Exposure Notifications

Continue → Select region - > Agree to PHA terms , Agree to enable EN