I don't want to delete no-ip's account or any info there. I am replacing my MR400 with a MR600 router, so the no-ip credentials in the old router MR400 are now obsolete. If I don't delete the credentials in MR400, there will be two different routers that update no-ip's dns with same credentials. I solved it by setting old router's credentials to no-ip as nonsense, even it complains when saving them in router's admin page.

Nope, I did a logout, but after next boot the router is back logged-in to no-ip.