Having a Device associated with a Subscription doesn't impact connectivity.

The only time you need to have a Device associated is for any users on AT&T SIMs - this relates to AT&T whitelisting requirements, meaning they need to know and approve specific devices to be able to use their network. Entering the IMEI is already part of the activation flow for these SIMs.

Adding a Device is not necessary for T-Mobile SIMs, so it's normal to see these subscriptions without an associated Device.

For any situations where you want to add a Device please follow these steps. This is useful for example when you're exchanging a T-Mobile SIM to an AT&T SIM, or if you want to add it for your records.