There's no !set regmax, you probably mean !set maxusers, but it will not make the address unreachable or anything.
If there's an 'address unreachable' or something, then the hub isn't running that port or the dns address is pointing to the wrong IP perhaps. If you're running a secure port, then a message like that may appear when trying to connect to it without a secure connection. It's not a message FlexHub will send to connecting users.