It did work with pbx in two years(not now). In your case, try to use ET263 to bridge your calls. Be carefull with your callout rule and use proper firewall to protect your goip.
In my case, I had to use extra antenna due to weak GSM signal. Heat dissipation might be your headache in sg. Use the inbuilt daily reboot function might be good when you cann't return to sg frequently.
If you use ET263 to bridge your calls, there is not need for fix-ip nor ddns.
However, if your pbx has restricted ip access rules or you need remote control over the goip, you might need fix-ip or ddns. I prefer access thru vpn with router rather than goip(risk for hacking).