I was able to watch it on other devices by going to www.peacock.com, and using the I don't know my password link to get an email to set a password. Then I logged in at their website with my email and the newly created password, and it is working fine.
I had a free Peacock account with the same email as on my Roku and I am able to watch Peacock on any device. Under account plan and payments it says (Billed by Roku.)
This may have changed since this thread was created, but like someone earlier in the thread mentioned, just do a forgot password, and set the password after you get the reset email. Worked fine for me and I can watch on all my devices, account says "billed by Roku".