China Mobile 移动王卡, I got it for app usage in Dec 2018. First year 50% OFF at RMB9 per month.
You can say it is the same as other mobile phone plans in China for all the major telcos there -- usually you deposited some money and top up regularly to maintain the balance. You can call it prepaid if using terms in Singapore. I was adviced to maintain RMB 200 balance to better use roaming outside of mainland China.
There should be similar plans from China Telecom and China Unicom as well. Cheapest may be below RMB10 per month.
China Mobile 移动王卡 RMB18 per month
https://touch.10086.cn/goods/220_220_1060520_1047966.html
China Unicom 腾讯大王卡 RMB19 per month
https://wk.10010.com/mobile.html#/
China Telecom 电信星卡 RMB19 per month
http://www.189.cn/products/90361274994.html?intaid=jt-sy-yd-05
All the above three can enable overseas roaming service to receive SMS in Singapore. If you visit China you can pay RMB 1 per day for 1GB data. So it is pretty convenient. And usually they will give you discount for the first year.
I will recommend the above three.
MVNO is even cheaper. The following is from JD at RMB 5 per month and using China Unicom network.
https://item.m.jd.com/product/4006241.html
But I am not sure whether it works in Singapore to receive SMS or not.
Edit to add: as per JD, you can add overseas roaming as well for the RMB5 per month plan.
https://item.m.jd.com/ware/consultationDetail/2637681-63297731.html
Xiaomi Mobile offers a low cost RMB6 per month plan.
https://partner.10046.mi.com/miMobileOrder/card_sell?channel=9999&from=juhe__main_home