Environment
Device: applicable to all
Question
Can I leave the APN blank?
Can I leave the APN set to default?
Answer
Yes, you can, but it is not guaranteed that your device will be able to access the Internet.
Set the APN to: onomondo (all lower-case) to avoid issues.
Here is an article on how to set the APN.
Cause
APN (Access Point Name) is a necessary prerequisite for Internet access. While some LTE networks may be able to deliver the right APN to your device, it's best to manage this from the device side and not rely on the network.