You have to let Apache know where the CertificateFile and CertificateKey are and reload Apache.
Edit /etc/apache2/sites-enabled/nextcloud.conf to include:
SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/my.domain.tld/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/keys/0000_key-certbot.pem
Edit: should report issues snap/https here