Port 443 SSL issue with modem only - 4G is fine

[crmpicco]

I am on MacOS Monterey 12.5.1 (21G83) and I am having difficulty connecting to an endpoint with SSL via a VPN (required) 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 and a factory reset to no avail.
I have disabled IPv6 so it doesn't get in the way.

 

[bogdanw]

Disable the firewall on the modem.

 

[Sheepish-Lord]

Have you tried not using the vpn? Seems like the server you are connecting to has a valid certificate but your device needs to handshake successfully which it’s not. This wouldn’t typically be a modem issue unless you’ve done something specifically to it or have security restrictions enabled.

Reset the modem to factory and if that doesn’t work look at the vpn.

 

[satcomer]

This error comes up with Locked files files too! Some devious developers lock files trying to keep their program on user computer! However just changing the premission on the file through a 'Get Into' key combination to change a file's permissions!

 

[crmpicco]

Disable the firewall on the modem.

Yep, i've done that - sadly no effect.

 

[crmpicco]

Have you tried not using the vpn? Seems like the server you are connecting to has a valid certificate but your device needs to handshake successfully which it’s not. This wouldn’t typically be a modem issue unless you’ve done something specifically to it or have security restrictions enabled.

Reset the modem to factory and if that doesn’t work look at the vpn.

Yes, I have removed the VPN from the equation by (temporarily) opening the server up to the world. The VPN is having no effect here. I've also done a full hardware reset of the modem too.

 

[belvdr]

What DNS servers are you using when connected to the modem? Some security DNS services, such as OpenDNS, may be detecting the FQDN in a blocked category and attempting to block access. I would try using Google's public DNS servers after connecting to the modem (8.8.8.8 and 8.8.4.4).