Does the unit I want to rent need to be a legally registered residence in order for me to get renters insurance coverage?

Most likely, an insurance company is going to want the address to be verifiable before they offer coverage. If the unit doesn't have an address, you should list the main residence's address on your renters insurance and the same rules would likely apply. Most renters plans cover your personal property and personal liability, so where you are living at the residence should not matter to the insurance company.

Thanks and have a great day.