I think you have set up everything correctly but the issue with audio and dropped calls could be due to a few factors
You should Double check your NAT and firewall settings. Ensure RTP ports are properly forwarded and there is no interference from the MikroTik router. Also Confirm SIP ALG is completely disabled on your router, as it sometimes re-enables itself after a reboot. Review your Kamailio configuration to ensure it handles NAT and RTP correctly. You might need to tweak settings related to NAT traversal. Please verify that both Kamailio & FreePBX have matching codecs and RTP port ranges configured.