I managed to get the secure MQTT working. Got a  tip from DELL WMS documentation and the solution were to enable local firewall on the WMS host server and block the incoming traffic to the non-secure port (TCP 1883 in my case). After that the ThinOS went automatically over to secure MQTT (TCP port 8443 in my case).
Could anyone possibly share some insights on how to configure Wyse ThinOS clients to talk with WMS over secure MQTT channel? My test WMS has both the TCP ports 1883 (non-secure) and 8443 (secure) open but the ThinOS client shows it's talking only over non-secure 1883. So how to get the ThinOS clients by-default to choose the secure MQTT over the non-secure?   Thank you for your help.
How to configure Wyse ThinOS clients over to secure MQTT channel?