I tried that when I first got here last year. I’ll spill the beans, you simply can’t do it. Despite both Mastodon and Lemmy supporting the ActivityPub protocol, they are different platforms. You can exchange posts and comments and whatnot, but you can’t use an account from one platform to log into another.

Just like email, you can’t log into Proton Mail with a Gmail account, and vice versa, despite both platforms being able to exchange emails (because duh, that’s how email works).

I hope that explains everything.