Many sheng siong stores including Bedok central, Clementi west, Bukit batok, Clementi Ave 3, etc
ABC store in Bedok central
Go try SS Commonwealth. In the past no Singtel network unless near the cashier counters/entrance. The other day, no network at all even when I was there (checking out the Want2 bags).
Simba had connection all the way inside.
Edit:
@mosmos I only noticed Simba had connection because I'd my AirPods on one afternoon right inside, when I took out my phone to check something online. Then I realized my music wasn't cut off (unlike previously with Singtel line), and I could also check my online stuff. (My Hotlink card roaming on Singtel had no line.)
The 2nd time I went to check on Want2

I was near the cashier counters when I took out another phone to check something then realized no line. Yup, Singtel line. So I'd to dig into my backpack for my Simba-line phone.
A couple of friends living in the area also said their phones couldn't be used in there. "Pioneer" Singtel users.