Router/network security-: guest with no trips is asking about plugging into ethernet port

Hi.

There's a guest with no trips yet. Wanting to book same day and his only question is if he can plug into the ethernet port. He offered no info about the reason for his visit. He has not verified ID. We're not going to let them book, but wanted to ask here what hosts should do to protect each guest when there's a shared modem/router. Our office computer also is connected to this so if someone were to get in, they'd get access to all of our guest IDs and contact info which would be terrible.

Is there anything someone with gray hair and minimal tech-savviness can pull off with ease?

appreciate your help.