Hi,
cant really help more than others comments for double network related issue.
for provisionning only :
I have encounter similar problem regulary, asked on this forum and someone pm me a solution which solve the problem in my case.
When you do "netstat -g" on your 3CX terminal, if you dont see sip multicast running as service, you need to go from 3cx main page into "services" and restart the sip server. After that you can check sip multicast is running with same command as before and provisionning works.
It solve the issue for us, we have to do it for every install, 3CX is on a debian NUC, NUC not supported by 3CX (3cx only support older NUC).