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:  

That is not correct. Actually, the NS records for your domain are exactly the same ones you could add for a subdomain, just in the root zone. And even tho its not his intention, yes, this would work for clients too. For DNS, there is not really any difference between example.org test.example.example.org as long as you set NS records. Only cloudflare blocks you from adding such a subdomain to their network. If you e.g. set NS records, you can host a subdomain with its own DNS entirely as you wish on dns.he.net  (last time i checked they allowed domains not on the PSL). DNS doesnt care about the PSL. Its mainly cloudflare that does, and webbrowsers.   The CNAME record you added only verifies that the domain is allowed to be added to byet hosting. However, you still need to setup DNS records. In Cloudflare, create following records: hosting.cheater.de A 185.27.134.175 www.hosting.cheater.de A 185.27.134.175   This IP might change in the future. You can always see the IP here: https://digwebinterface.com/?hostnames=hosting.cheeter.de&type=&short=on&useresolver=9.9.9.10&ns=self&nameservers=ns1.byet.org That is also why its reccomended to set the NS records.  

i dont wanna use this domain for client accounts, basically i have a domain "site.cheeter.de" to register (username.site.cheeter.de) and i wanna use hosting.cheeter.de only as the homepage, i did it this way so clients cant register hosting.cheeter.de. i then tried adding the hosting.cheeter.de via CNAME which worked but didnt show any content on the page.

That domain is not on the PSL. While you can add DNS NS records, you can’t add nameservers records (which are what is required). Even if you could get it setup with the domain, you certainly would not be able to use it for client accounts as any domain users create would not work.    just buy a separate domain for the hosting platform or use the base domain name. The system is a bit rickety, doing something wrong will probably break something major at some point. 

The Support just says i need to add the Domain "cheeter.de" for this to work but it worked before without that domain so i think it should also work this way. before i did it with "hosting.cheeter.de" as NS but deleted it for reasons, i wanted to add it with CNAME which worked but didnt show anything like said before.