What do you mean by "activated under my plan"? If they are all linked to your Roku account at
https://my.roku.com they can all be used simultaneously. Note that some channel subscriptions may limit how many simultaneous streams you can receive (e.g my Netflix account is good for two, my Acorn says it's limited to one but I have been able to use two).
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Do not ask me support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.". Please only do this if it ACTUALLY solves your problem.
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X | Streaming Stick 3600