Please make sure that you change the nameservers of your domain name. Note that this is generally a different setting than the DNS Records setting. Changing nameservers is NOT done by editing the NS records of your domain name.
And this @HaydenANG is why doing a dig ns is not reliable. Only a dig +trace works.
But the Domain Checker not returning nameservers is not correct. Will look into that.
TL;DR: You didn’t set the nameservers on your domain.
Again, looking up the NS records doesn’t work. It’s equivalent to doing dig ns, which only tells you the NS records returned by the domain’s nameservers, which may not necessarily be the nameservers that the domain name is actually using.
Again: changing the nameservers of a domain name is NOT done through the DNS Records interface. It’s always a different setting.
And if your domain registrar does not allow you to change the nameservers, please consider switching to a registrar which does let you configure the domain the way you want to.