Accidentally restored a backup on an old phone and can't logout from iCloud

Hi guys, I think I've made a mistake.

So I used to work for this company a few years ago where I had an iPhone, using my work email address as an AppleID. I made a backup to my computer with as I had dozens of private photos etc. on that phone. I forgot about it and today I made use of an old iPhone 7 that was available for use and restoration.

I restored the backup on that phone and I sent all the necessary photos while staying offline. But the problem occurred when I wanted to erase the phone. I cannot log out of the iCloud (that I didn't really login to) as I don't have any password to do so. The email address I was using at that company for all I know is inactive so there's no way to retrieve a password, not that I'd want to contact them about it, as they might feel like I've been snooping around or something.

Can I somehow skip the logging in to log out? I just want to erase the phone and bring it back to factory settings.

Thanks in advance!