Singtel bundled router has always been the worst that they can scrimp. Since 20 years ago.
Maybe you need to qualify this with "free bundled router" for the "usual Internet plans" or "base plans"
There are good routers which are bundled with Singtel Fibre Internet now. For example, they bundle Asus RT-AX86U with the 1Gbps Pro Gamer Plan and 1+1 Gbps Game Bundle plan.
https://www.singtel.com/personal/products-services/broadband/fibre-broadband-plans
And sometimes it may not be the worst. Nokia Beacon 1 bundled with Starhub 1Gbps plan (which was a downgrade from the earlier Linksys EA7500v2 and Linksys EA8100 v1) a few year ago may have a good fight with Singtel Mesh Router to be the worst. I will argue Singtel Mesh Router is better in terms of wireless performance.
I do agree that Singtel has bundled many lousy routers -- I have been with Singtel since Feb 2012. I would say the 2-wire and Pace router were the worst routers back then -- I have to change to Linksys WRT1900AC and never look back at Singtel bundled router myself. Singtel AC Plus is also the worst router at its time (2018-2019).
However, since Starhub switched to Nokia Beacon, I think Starhub has a good fight with Singtel to bundle the lousiest router for the base 1Gbps plan.
Singtel Mesh Router (2020-2021) is so-so but Nokia Beacon 1 is very bad as well.
As for the current Singtel issued free router (Airties 4960R AX5400 router, or Askey vs Starhub issued Nokia Beacon 2 AX1800, I am not sure which one is worse. Both are lousy any way.
M1 seems to be better with the free bundled router for 1Gbps plan: Asus RT-1200G+ was so-so at its time, Asus RT-AC2600 was pretty decent, Asus RT-AX55 or TP-Link Archer AX72 is quite good compared to Singtel/Starhub's offerings. But I would still suggest to get better ones like GT-AX6000 from M1 if you are an M1 user.
In any case, DO NOT use the bundled router as the main criteria to choose ISPs. In fact, Starhub is still my top recommendation along with M1 for new sign-up. You just need to skip the Nokia Beacon.
https://forums.hardwarezone.com.sg/threads/singapore-isp-comparisons.6665380/