I don't know MikroTik router, sorry.
In any cases, you should have some parameters to forwarding some ports (sip and rtp under UDP) to a specific IP address.
About Elastix, like i said :
..download and read the documents Elastix Easy and Elastix Without Tears for the common configurations..
There's several threads on this forum about NAT configuration. Start to read them. Next, if you have any problems again, let me know.