Any 4G+4G or 4G+3G smartphone will only have the line that is on active voice call to be connected to the network. The another line will be put on hold.
The only exception is when your iPhone is using WiFi Calling, then both lines are still connected to the network.
If your main voice line is SingTel and data line is Circles, whenever you receive the voice call on SingTel, if the "Allow Mobile Data Switching" option is off, you will not be able to data at all. It will only recover after your call has ended and reconnected back to Circles.
Also, if your voice line, for whatever reason, dropped back from 4G to 3G during the voice call (SRVCC), the iPhone may be cut from the internet until you airplane mode and un-airplane mode. That happened to me a few times before, but idk now with iOS 13.6.1/13.7 if the issue is still there.