To set up the HTTPS protocol for a subdomain assigned to Tilda, add (or change) the settings of DNS records (not NS servers) in your account on your registrar's website:
Note. There should be no more than one type A record for "@" and one for "www".
Once it's done, you may need to wait for up to 24 hours for the DNS records to be updated (depends on the registrar) and connect HTTPS in Site settings → SEO → HTTPS Settings. Click the switch button. HTTPS will connect automatically.
Note. It is quite common if a subdomain points to Tilda while a root domain remains connected elsewhere.
To set up an SSL certificate for a subdomain assigned to Tilda, go to the website of your domain registrar (such as GoDaddy or Namecheap.com). In the Control panel find Zone management menu and modify A records of a subdomain.
Learn more about HTTPS: http://help.tilda.ws/https