Hi Yeastar Support !
I already found similar issue here: https://support.yeastar.com/hc/en-us/community/posts/115000745107-SIP-Trunk-Issue
In my example, we have S20. Each WAN failure or even manual restart of the main router cause isse that TRUNK is going into "not registered" solution is to restart S20 or re-register TRUNK.
Please advise.