For ISP issued fee routers, Starhub issued Linksye EA7500 v2 and Linksys EA8100 v1 have good support of OpenWRT now. Unfortunately there are no support of the popular Starhub issued D-Link DIR-868L (even if there is support WiFI will suck due to the use of Broadcom chipset). There are snapshot release for Starhub issued DIR-865L and DIR-890L but wireless will suck because of no good 5GHz band support.
I tend to belive that you can run OpenWRT on M1 issued RT-AC2600, by using RT-AC85P OpenWRT firmware (double amont of RAM of RT-AC85U/85P). Take note there is no support for the older RT-AC1200G+.
https://openwrt.org/toh/asus/rt-ac85p
Some of the other local ISP issued routers may also be supported by OpenWRT, like Linksys WRT32X (M1 gaming plan).
As for Singtel issued free routers like Aztech, Arcadyan, Airties or Askey, they will have basically no chances to get OpenWRT support.
Singtel issued free routers for Fibre Internet
2-wire 5012NV --> Pace A5520N --> Aztech DSL7003GRV(AC) --> Aztech DSL 7009GR(AC) --> Singtel AC Elite (AC1900) --> Singtel AC Plus (AC1200) --> SIngtel Mesh Router --> Airties Air 4960R -->Airties RT5703W (now)
Singtel ONR users may get mesh nodes like Singtel Mesh Extender and
Singtel AC Plus (AC1200) --> SIngtel Mesh Router --> Singtel Mesh Extender or Askey AP5100W WiFi Mesh (mesh node, not router) --> Singtel WI-FI 6 MESH AP5690W (not a router) or Airties RT5703W
Starhub issued free routers for Fibre Internet (D-Link models up to July 2017)
?? --> D-Link DIR-865L (2013-2014) --> D-Link DIR-850L --> D-Link DIR-868L and D-Link DIR-890L (for upgrade?) --> Linksys EA7500v2 --> Linksys EA8100 v1 --> Nokia Beacon 1 --> Nokia Beacon 2 (now)
M1 issued free routers for Fibre Internet (?)
RT-N15U --> RT-N56U --> RT-AC56S --> RT-AC68U (not free, for upgrade?) --> RT-AC1200G+ --> RT-AC2600 --> Asus RT-AX55 or TP-Link Archer AX73 (short duration) or TP-Link Archer AX72 --> RT-AX3000P or TP-Link Archer AX72
