nginx listening on ipv6, but for some reason websites won't load

I'm trying to get ipv6 working on nginx (nginx/1.19.6) for SSL. In my domain configs I have:

server {
    listen      93.93.135.169:443 http2;
    listen      [::]:443 http2;
    server_name backups.myserver.com;
   ....
}

..and then for the standard port;

server {
    listen      93.93.135.169:80;
    listen      [::]:80;
    server_name backups.myserver.com ;
    ...
}

Netstat shows nginx as listening on port 443 and 80:

 netstat -tulpn | grep nginx
tcp        0      0 93.93.135.169:80        0.0.0.0:*               LISTEN      1168/nginx: master
tcp        0      0 127.0.0.1:8084          0.0.0.0:*               LISTEN      1168/nginx: master
tcp        0      0 93.93.135.169:443       0.0.0.0:*               LISTEN      1168/nginx: master
tcp        0      0 0.0.0.0:9183            0.0.0.0:*               LISTEN      5247/nginx: master
tcp6       0      0 :::80                   :::*                    LISTEN      1168/nginx: master
tcp6       0      0 :::443                  :::*                    LISTEN      1168/nginx: master

I can find the server find on ipv4 and ipv6:

root@admin3:~# ping -4 backups.myserver.com
PING backups.myserver.com (93.93.135.169) 56(84) bytes of data.
64 bytes from backups.myserver.com (93.93.135.169): icmp_seq=1 ttl=60 time=1.58 ms
^X^C
--- backups.myserver.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.575/1.575/1.575/0.000 ms
root@admin3:~# ping -6 backups.myserver.com
PING backups.myserver.com(2a00:1098:80:a0::1 (2a00:1098:80:a0::1)) 56 data bytes
64 bytes from 2a00:1098:80:a0::1 (2a00:1098:80:a0::1): icmp_seq=1 ttl=61 time=1.55 ms
^X64 bytes from 2a00:1098:80:a0::1 (2a00:1098:80:a0::1): icmp_seq=2 ttl=61 time=1.74 ms

On iptables, I get:

iptables --list -n | grep 443
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 80,443

iptables --list -n | grep 80
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 80,443

..and ip6tables:

ip6tables --list -n | grep 80
ACCEPT     tcp      ::/0                 ::/0                 state NEW tcp dpt:80

ip6tables --list -n | grep 443
ACCEPT     tcp      ::/0                 ::/0                 state NEW tcp dpt:443

Yet , when I test the site on https://ipv6-test.com/ with http, it works:

But as soon as I try SSL, I get:

Running a curl test from another server to it, I get:

curl -v -6 https://backups.myserver.com
*   Trying 2a00:1098:80:a0::1:443...
* TCP_NODELAY set
* Connected to backups.myserver.com (2a00:1098:80:a0::1) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* error:1408F10B:SSL routines:ssl3_get_record:wrong version number
* Closing connection 0
curl: (35) error:1408F10B:SSL routines:ssl3_get_record:wrong version number

I'm running out of other things to try. Any suggestions?