Device A: BTU = Huawei HG850a ™ <- Can't Access via webpage since TM Installer modified it with Firewall
Device B: Residential Gateway Router/Modem = Technicolor TG784n v3 [upgraded version of Thompson TG784n] (Maxis)
Device C: D-link 615 (Flashed firmware with DD-WRT interface UI) (UNIFI-TM)
I recently terminate my UNIFI & subscribe to Maxis Fibre.
They setup Device A(LAN 2 Port) + Device B (WAN Port) for me;
but I intend to use back the D-Link 615 (Device C) to replace the Device B.

*Warning: You will NOT able to use Maxis Dect-Phone VOIP Service with this setup.
My Final Setup
Device A (LAN 2 Port) + Device C (WAN Port)

GUIDE: http://unifi.athena.my/ddwrt_guide.php  *vlan2 change to '621' for Maxis

Basically, concept is the same with UNIFI, just the username+password & vlan tagging number changed.