I was viewing it from an iFN perspective, they require their nameservers to be set on the domain in order for it to be added. I also read the OPs statement incorrectly so that contributed to by incorrectness as well. But yeah, DNS NS and nameservers are the same thing, but PSL does matter when a service restricts you down to only adding a root domain name. I thought I remembered someone coming to a conclusion that iFN did do PSL checks for some free hosting stuff (And very badly tried to communicate that in my last post).
OP, read this, it is correct and is your next step assuming that the domain is actually connected. If it still does not work after 72 hours / DNS cache clear and verification, see my above speculation about iFN and PSL: