IFastNet is a diferent company to InfinityFree, so most of us probably wont be able to offer much advice.
Do IFN have their own forum? That might be the best place to ask. As for the login issue, you may want to reach out to their customer support for that.
Once you’ve got the domain with with IFastNet if you want to add it to your InfinityFree account this link might help - How to add your own domain to your account