SSL handshake error when connecting to Chef Automate server via modem

Chef Automate
1

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 prod-hjsxv.us-east-1.opsworks-cm.io:443
ERROR: The SSL certificate of prod-hjsxv.us-east-1.opsworks-cm.io 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 prod-hjsxv.us-east-1.opsworks-cm.io:443
Successfully verified certificates from `prod-hjsxv.us-east-1.opsworks-cm.io'

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 prod-hjsxv.us-east-1.opsworks-cm.io:443               
CONNECTED(00000005)
4305192492:error:140043E8:SSL routines:CONNECT_CR_SRVR_HELLO:reason(1000):/AppleInternal/Library/BuildRoots/20d6c351-ee94-11ec-bcaf-7247572f23b4/Library/Caches/com.apple.xbs/Sources/libressl/libressl-2.8/ssl/ssl_pkt.c:1200:SSL 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/com.apple.xbs/Sources/libressl/libressl-2.8/ssl/ssl_pkt.c:585:
---
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
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : 0000
    Session-ID: 
    Session-ID-ctx: 
    Master-Key: 
    Start Time: 1662019305
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
---

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