@Quann_SG, I don't know if they would allow you to use more than one domain, since the product page states you can have one website (one domain is what I read).
@Quann_SG What do you mean by linked to client's domain?
Is the hosting account for a client of yours? You can check what domain it's showing in the gateway.
You can also change it, by clicking in Settings and then choosing the domain you want to that hosting plan.
If that doesn't work for you, you should contact support.