Before transferring my domain to Infinity Free I used to be able to access my website via a browser using just the domain name but now I have to append it with /index.html - why is this?
You will have to releases your nameservers. There’s no EPP code because this is not a domain registrar. Once you release your nameservers and A records, you are free to use your domain elsewhere.
If you have a subdomain from InfinityFree, that is true. But you do not, you have a custom domain registered elsewhere that’s hosted here at InfinityFree. Registrars and Hosting providers are different things. If you follow my guidance above, you will have that domain ready to host a website somewhere else.
We don’t control your domain registration. We cannot “release” your domain or provide you with an EPP code, because we don’t have that control. Only you can do that.
You can also change the nameservers of your domain name to point them to another hosting provider. That’s the way to stop hosting your domain with us.