[RobbieRobski]

On wp8.1 cortana didn't need internet connectivity in order to add reminders. However on w10 mobile this appears not to be the case. As I am learning due to my current stint with tmobile, no connectivity = no cortana. And no, I'm not an insider.

[spicypadthai]

Yes, check out the Cortana and privacy page. If you aren't connected to the internet, you can't sign into your MS account, and based on the privacy summary "your experiences will be more limited and they won’t be personalized with your Cortana data." While the summary doesn't go into details of Cortana data vs. non-Cortana data, reminders are clearly Cortana data since they can be shared across devices.

[RobbieRobski]

On one hand I get it. Because I remember setting a reminders on wp 8.1 it wouldn't remind me on windows 10. But this is something they could easily fix by simply syncing the next time you connect to a network.