One Subscription for all devices
Yes! One subscription is good for all of your devices. You just need to make sure you are logged in under the same account on all devices.
I logged in with Facebook/Google/Apple, but don't have access on my other device?
On the device that you are logged into, go to the Settings (app's menu > Settings) and select Add/Update password to create a email/password combination that you can use on the other device.
Apple users: Double check the email address that is being used. When you sign up using your Apple ID, you have the option of "Hide My Email". Your account is then created under an anonymized email address that looks like this: firstname.lastname@example.org . You'll need to use that anonymous email address to log in, or you can change the email there in the Settings.