Actually, the good news is that you didn't get banned. I wouldn't even try to refer one of my neighbors because it might look like multiple accounts because we all subscribe to the same ISP.
It might be okay; however, I like my FusionCash account so I won't take a risk.
You should be fine referring your neighbor. Sharing the same ISP is not an issue. You each should have your own assigned IP from the ISP that makes your account unique (in a manner of speaking, that's how the ISP is able to bill you correctly). As long as you are not sharing the same wireless service, you are good. If you each get your own bill from the ISP, and are using your own connections - you will both have your own digital signature.
If the situation is you are in an apartment that provides your internet service as part of your rent, and everyone connects to the same wireless internet; then you would be correct - unless you contact FC support to see if they are able to distinguish accounts in situations like this.