I have seen many posts online regarding disabling of SIP ALG on routers. Strange thing is, after I have disable SIP on my EdgeRouter, the remote IP-phones failed to connect to 3CX PBX.
What problems were you having before you disabled SIP ALG? How are your ports forwarded for your 3CX server? Are you doing 1 to 1 NAT, sounds like maybe ports are being changed by the router. Could be a lot of things, what are you seeing in logs on the edgerouter? Are you using Unifi phones as well that are managed through the unifi software?