Monitoring doesn't need write access. RS-422 is simple and reliable. It uses separate receive and transmit pairs and you don't need to connect both. The freezer can continually transmit its telemetry over unidirectional RS-422 and a separate system can forward it over the Internet.

Hackers can still tamper with the telemetry and create false reports of problems, but actually changing the thermostat settings would require local access, so there's opportunity for somebody to verify the problem really exists.