"Remote Managment" Username and Password for unlocking an old iPhone 7.

Whoever installed the MDM software on the phone is who you need to contact. Apple wouldn't have access to anything associated with that software. Without it, you won't be able to do much of anything with the phone. Contact either the school or employer who put the software on your phone.

Sounds like the phone belongs to an old employer, or you enrolled it in your employers management system.

There's no way past that.

Honestly, at this point that phone isn't worth the trouble of resurrecting. Even if you manage to the the company that has it under management to release it, the battery has to be on its last legs. It will cost more to replace the battery than that phone is worth, if you can even get it replaced.

That device is considered 'vintage' by Apple. Parts and service availability are very limited now.

Just guessing, but it sounds like the phone was managed by an organization, and you have encountered a variant of Activation Lock.

If you don't know the username and password, there's no way that we, your fellow users could be expected to know.

With proper proof of original purchase, Apple can remove Activation Lock on phones that are not in Lost Mode. But my guess is that they will refuse to unlock any phone that is under organizational management unless the request is approved by the organization (if then).

It seems likely that this iPhone 7 will forever remain a brick.