SSL handshake error when connecting to Chef Automate server via modem

I am having difficulty connecting to my Chef Automate server (hosted on AWS OpsWorks) using my modem. When connected to the modem I get the following error, whereas if I switch over to tethering on 4G it works fine.

On my Telstra Arcadyan LH1000 smart modem gen 2:

Connecting to host
ERROR: The SSL certificate of could not be verified
ERROR: Could not establish a secure connection to the server.

Disconnect from the modem, tether on my phone on 4G:

Connecting to host
Successfully verified certificates from `'

I feel like this is a modem issue, but I can't put my finger on the problem.

I have tried the usual troubleshooting techniques such as reboots, disabling modem firewall and a factory reset to no avail. I have disabled IPv6 so it doesn't get in the way.

openssl version
LibreSSL 2.8.3
openssl s_client -connect               
4305192492:error:140043E8:SSL routines:CONNECT_CR_SRVR_HELLO:reason(1000):/AppleInternal/Library/BuildRoots/20d6c351-ee94-11ec-bcaf-7247572f23b4/Library/Caches/ alert number 0
4305192492:error:140040E5:SSL routines:CONNECT_CR_SRVR_HELLO:ssl handshake failure:/AppleInternal/Library/BuildRoots/20d6c351-ee94-11ec-bcaf-7247572f23b4/Library/Caches/
no peer certificate available
No client certificate CA names sent
SSL handshake has read 7 bytes and written 0 bytes
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
    Protocol  : TLSv1.2
    Cipher    : 0000
    Start Time: 1662019305
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)

Is this an issue with my device, modem or server?