Richard,
Seems from reading this post that TLS support depends on the Java version you are using: https://forum.mendix.com/link/questions/18192 As long as you are using Java 8, TLS v1.2 is supported.
Hope that helps,
Mike
Hi Richard,
To add to Mike's correct answer, in the cloud portal you can set the java version in the 'Deploy' tab
However, Mendix 6.10.3 requires that you use Java version 1.8, so you won't need to set any custom settings for TLS v1.2 to be supported in your application.