If you install the iOS 11.3 beta without registering your UDID, your device will be deactivated with an error like this:
This device is not registered as part of the iPhone Developer Program. If you are a member of the iPhone Developer Program, please register your device in the iPhone Developer Program Portal.
As Apple warns, installing beta software on your device without registration "could put your device in an unusable state and necessitate an out of warranty repair." By registering your device, you'll be able to safely use any beta firmware.
In the event of a wait list due to high demand or Apple downtime, we will register devices with the Ultimate and Premium packages first.
Our support team gives higher priority to customers who purchased the Ultimate and Premium packages.
If your device is replaced due to theft or exchanged under an insurance program within 6 months of purchase, we'll register your new device. This is a free service for those who purchased the Ultimate and Premium packages.
Apple can revoke certificates at any time and for any reason. While the Plus package is only eligible for replacement in the first 90 days, the Ultimate package remains eligible for replacement for an entire year. Simply look up your order and download a replacement certificate. Completely free and automatic.
Follow these instructions to find your UDID.
Enter your UDID on the homepage and click "Check" to see the status of your registration.
We send confirmation emails instantly to the email address associated with your PayPal account. Be sure to check your spam folder.
Our service doesn't provide you with direct access to a developer account or beta downloads. All we do is register your device with one of our own developer accounts.
Your registration will last for all betas of iOS 11.3.
Follow these instructions to enter recovery mode, then restore to iOS 11.2.5.