Checking the DNS to make sure the original domain is routed to your server is always a good first step.
If so, I like to then use Curl to see if the redirect happens. For example, this output shows a domain redirecting from http to https:
$ curl -v example.domain.com
* Trying 18.104.22.168...
* TCP_NODELAY set
* Connected to example.domain.com (22.214.171.124) port 80 (#0)
> GET / HTTP/1.1
> Host: domain.com
> User-Agent: curl/7.64.1
> Accept: */*
< HTTP/1.1 301 Moved Permanently
< Date: Tue, 03 Nov 2020 15:31:16 GMT
< Server: Apache/2.4.46 (cPanel) OpenSSL/1.1.1g mod_bwlimited/1.4
< Location: https://example.domain.com
If your output shows something similar then the redirect is happening on your server. This could be from the .htaccess file or even in a database entry, such as the siteurl value in a Wordpress database.