How was you Max account set up and paid?
If you had an account directly with Max, then you should be able to log in via Roku or any other platform.
On the other hand, if you used some platform payment system (Google, Apple, Roku etc.) those platform payment accounts typically only work on that specific platform.
So if you have one of those platform accounts, you probably want to cancel that and go direct with a Max account that you can log in and manage on the web. Then log into that on your Roku.
Banned but back. Because why not?