OLT(NLT Fiber port)-----ONU-----Router
Yes I use Huawei OptiXstar MA5671A to be my customized ONT device. There are also similar Nokia and Hisense ONU which does similar job
https://e.huawei.com/sg/products/enterprise-transmission-access/terminal/onu/optixstar-ma5671a
The GPON ONU serves as a mini ONT/ONR device to do authentication and bridgeing. Singtel only authenticates user by LOID (Fiber broadband ID) and hardware SN (on the issued ONR). MAC is not required but I changed MA5671A's MAC to be same too.
Why such set-up:
1. to bypass Singtel ONR which defaults router mode, get rid of TR-069 and limited Gigabyte LAN connection
2. to free some
weak current box space, no additional power supply required to ONU
3. supports GPON max bandwidth single port, Singtel 1Gbps plan gives 1.2Gbps/1.0Gbps DL/UL speeds. 2Gbps Plan also benefits with 2.1/1.25Gbps speeds.
4. SFP+ supports up to 10Gbps speed. (There is XGS-PON ONU from Huawei but it has quite higher temp and not necessary as the ISP issued onr is good enough)
There will be many new ONRs coming out this year with 2.5G LAN port like cheap VSOL V2802RH, and old but more expensive ZTE F7607p.
Also, the reason why Singpoare ISPs market their 2Gbps plan with 1+1Gbps connection is simply onr hardware limitation, they just lazy and mean to provide newer ONRs with 2.5G port since most people dont actually care.
Seems like you always overestimate the complexity of Singtel VLAN. All ONU device can pass through or set VLAN, no matter it is ONR or mini PON device. Setting up Internet and IPTV on MA5671A is simply copying numbers from issued ONR, Voice would be a bit harder though.