I'm further away from the causeway, a couple of days before the deadline already my old dual SIM phone became like single SIM. 

I think XR Note4 lets you use 3G on both slots, no sure... but with the older phones you'll have to choose which one you want to turn off.

If you need to visit Singapore frequently and plan to buy a new phone, you'll have to be careful what phone you choose. I think not all new phones can support the use of 3G on both slots, or 4G on one and 3G on the other.

Yes, Maxis' roaming partner is Singtel but regardless, any Singapore network you put to 2G will not get a connection anymore...

2G have nothing to do with super super old phone lah. 2G have stronger signal and wider coverage than 3G. Take a look telco coverage map of Sarawak. People over there also human being too.  Better 2G than No Service bro
And u must be not using GPS tracker.