MulitTech certificate "Valid From" time set based on modem time.
User was getting "Could not establish trust relationship for the SSL/TLS secure channel" error when testing HTTPS connection.
User was on Windows 10, so it was unlikely to be due to unsupported TLS version. Double checked the steps to make sure the guide was followed properly (it was).
When looking at the rCell certificate from the Windows certificate viewer, we noticed that under Certificate Information, it said "This certificate has expired or is not yet valid."
Discovered that the modem's time was ahead of the the computer's time by several hours. When the certificate was generated, its "Valid from" time was also set several hours ahead, which made the certificate invalid.
Syncing the modem time and generating a new certificate with the proper start time fixed the issue.