With the version 18, 3CX decided to furthermore improve security and therefore ended the support of specific Cipher Suites that are considered Not Best Practise anymore. Additionally, v18 will simplify the DNS configuration with its DNS Helper feature. These changes prevent the M700 - a product that has been declared end of sales/support (EOS) June 2020, after 6 successful years in the market by us - to be auto-provisioned by the 3CX SIP server when updated to V18.
The only way an M700 base station can be provisioned on 3CX V18 is to switch off the DNS Helper feature, and use the provisioning method Local LAN. Any other provisioning method will request an HTTPs connecting to the 3CX server and therefore fail.