"Name server ns01.freenom.com does not answer queries over TCP."?

I've got a free domain from Freenom and want to connect it with Mailgun.

However, there is a DNS error with the code "ERR_NAME_NOT_RESOLVED".

I've tried to change the DNS on my computer as what most people suggested, or even cleared dns cache. But nothing has worked so far.

What could be the solution to the problem?

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Here's the link to the website : complain.ga

The error code:

Online DNS test tool result:

Here's the screenshot of my DNS setting:

Any advice would be greatly appreciated :) Thx!