I have been using AuthSMTP to forward emails from HMIs that can't do authentication. I don't use it as an SMS relay, I just program the HMI with the SMS gateway suffixes for all the common carriers, so the operator just enters his 10 digit number and picks the carrier from a drop-down list.
We have recently started using MQTT with some cell modems but I am not sure which SIM cards were bought. It is a company that offers them from multiple carriers and the data rates are quite cheap.
Another avenue for secure remote access to your HMI is a Stridelinx router with 4G modem. We have one of those (and a bunch of the wired versions) and let the end user pick the data plan, but it allows remote access to the HMI and PLC for us, and the HMI for the operators as well as allowing the HMI to send alerts.
