For WordPress to work properly on HTTPS, there are three steps.

1. Certificate and redirect

Make sure the domain has an SSL certificate and turn on the HTTPS redirect: see Plesk: Free SSL and redirecting to HTTPS.

2. WordPress addresses

In WordPress Toolkit, from the site card's menu click Update Site URL and enter the address with https://.

Alternatively, inside WordPress: Settings → General, change WordPress Address (URL) and Site Address (URL) to https:// and click Save Changes.

3. Old http links

If some images or files still load over http://, the browser shows the page as not fully secure. See My website shows as not secure.

Note: if a WordPress plugin does its own HTTPS redirect, keep only one of the two on, so they don't conflict.